Paradoxially though, the Republicans talk up a storm blaming mental illness and yet want to decrease funding for the very things that would do what they say they'd like to do.
Mental illness isn't responsible for school shootings. If nothing else, there are places on Earth other than the US that have terrible mental health services, yet we're the one place on Earth with recurring problem with school shootings.
IMO its challenging to attribute the rise in the 90s to any kind of motive increase. There was a crime wave in the 80s, attributable probably to either lead paint* or cops withdrawing from "bad neighborhoods". Since IIRC the early 2000s that crime wave has been receding, likely do to a reversal of either or both of those trends. So I don't think we can say that the jump in the 80s/90s is due to increased motive to carry out school shootings, because if mental health or general rage is the problem, it should have receded alongside the poisoning that causes brain damage and anger. The phenomenon of school shootings (in the indiscriminate murder sense, not gang violence sense) should have been present in the 90s mostly in poor inner city schools were social problems, lead poisoning, bullying and access to mental health services was the worst. So... why is it almost always white people from well off families? It doesn't line up with the mental illness or bullying theory.
If I have to point to anything it would be access to guns. To quote a long past Cracked article, you can't shoot up a school with a musket. The American civilian gun market is primarily composed of two groups: hunting and old west-esque guns (bolt action, lever action, revolvers, shotguns), and run off from the military's stock. The first group is difficult to conceal and not great for killing more than a couple people. The second group is perfectly fine for mass murder, but the civilian market is always a couple decades behind the military market. Some vets might keep their service weapons or similar for old time's sake but a military veteran will understand proper gun ownership.
So for purposes of school shootings, I'd say the civilian market is a good 20-30 years behind military small arms. In the 60s and 70s you have WW2 surplus, but you can't carry out a school shooting with a M1 Garand. And I could be very wrong (I don't know that much about the gun market, take anything I say about it here with a grain of salt) but I believe handguns were not standard issue for privates in WW2, only for officers.
So to my eyes what we're seeing here is school shooting effective weapons entering the civilian market. What weapon did we have in the 60s that's really deadly in civilian hands? Semi-automatic handguns. They flood the market in the 80s, and are made more mainstream and attainable by their popularity in the police force. US gun laws make it very easy for legitimate and "grey" market guns to slip through the cracks and fall into the hands of people without licenses. So presumably the amount of semi-auto handguns out there increases a little bit every year, as people buy them from gunshows or get them from friends/family, and they never are destroyed or go back into the legit market unless states set up a buyback program (which is rare). This would line up with school shootings increasing every year.
It would also line up with the lack of school shootings in inner cities; gun laws tend to be more strict in cities, and while professional crooks might have military surplus weapons, casual grey market gun ownership is very difficult for black people. Cause unlicensed gun ownership doesn't get caught on its own, it gets caught when cars and homes are searched for other offenses (usually drug related). Who gets randomly pulled over? Not white people. Additionally if your kid is going to a school infested with gangs and you leave a loaded glock in an unlocked drawer, its pretty clear that one way or another your child is getting taken away from you. So I imagine even a distracted/low-education parent in a "bad neighborhood" would avoid doing that. And obviously a drug dealer or gangster isn't going to let some 15 year old steal their gun, if nothing else because they need that gun to protect themselves from rivals. So while there are guns in the poor inner cities of America, there's not that same mass of unaccounted for grey market guns floating around where kids can easily find them.
Now this year we're seeing the effects of more modern military weapons entering the civilian market. The Los Vegas shooting and now this both used AR-15s. They were out there before, but I guess they were rare enough that someone who wanted to use them just happened to not get them. There's also less of an opportunity with AR-15s because they're almost impossible to conceal even with a backpack (presumably you could disassemble it but some snot nosed teen who isn't supposed to have it and doesn't want the FBI to catch them might not have looked it up), so that's even less opportunity for something like this to have happened. This has caused some renewed interest in the phenomenon of mass shootings, and its increased this years' bodycount to record highs, but at the end of the day it doesn't change anything. Enough people spray handgun fire into a crowd and they'll still make the bodycount go up each year; it'll be slower than what an AR-15 could do but its still unacceptable.
The other thing that I would point to is the copycat effect. News media gives legitimacy and voice to school shooters. Some angsty teen posts a manifesto on livejournal, no one cares. But if they go and shoot up a school, they don't even have to write a manifesto. Someone will write it for you and plaster it over every TV screen in America. As the fascination with killers increases, so does the glorification. Hence continuous increase over time in the number of school shootings. The other thing I would point to is Dick Wolf. All of his shows have a single, consistent narrative pounded in again and again over hundreds of episodes: "there are bad people in the world and they need to be punished." Its not even about criminals because a recurring theme in those shows is the bystander who the cops can't arrest but still has their hands dirty because they mistreated someone who went on to commit a crime. Its such an aggressive theme, and so prevalent across our media. Then you have reality TV, which has its own consistent narratives: either "the US has moral decay" or "the US is full of idiots." That's not necessarily the intended takeaway from the show but that's what people THINK about shows like Jersey Shore and Keeping up with the Kardashians and the showrunners started pandering to it pretty early on.
And so for someone who has that super repressed/angry "the world is full of corrupt sheeple and I'm the only smart/moral person, and I'm being wronged again and again" attitude, American TV and culture doesn't try to convince that person they're wrong. It tells them they're right. Our TV programming is telling our people "you should do something about all those dumb assholes in the world, and if you do, we'll hold you up as worthy of study." This lines up with the demographic of killers. What other group of people has cultural encouragement for mass murder? Islamic teenagers and young adults. ISIS puts up radicalizing info on the internet because they know someone will bite and they have experience radicalizing people on their home turf. Its not just that Islam has violent teachings; all Abrahamic faiths do. And its not just that Islam is a minority religion subject to discrimination and bullying. If that was the case, Jews should have been carrying out mass shootings throughout our history. They aren't bullied everywhere but they are bullied in some places. But its not about bullying. The radicalizing materials are are out there for Islamic and arab individuals and mainstream American audiences are not receptive to that. This lines up with the Charleston terror attack; white nationalists had their own radicalizing materials.
And so the way I see it, what you need is two things. You need access to guns, and you need radicalizing materials. Cause here's the thing. There are countries in the "third world" where black market gun ownership is common, in fact their are plenty of places on earth where the government cannot keep order and the standard is that local militias and neighborhood watches take the place of a police force, and they're generally armed. The entirety of the "third world" has become armed, its about the only thing Metal Gear Solid gets right about world politics. So why don't we hear about the school shooting in Ethiopia? Like there are stable developing nations out there where gun access is very possible, yet they aren't a warzone or failed state. And you still don't see kids shooting up schools in those places. And yeah that's possibly because people are a little better about gun storage. But I doubt that's the whole story. The way I look at it, those people don't have commonplace internet access. People who live in rural areas without access to modern infastructure take their cultural and moral cues from their family and community. Morality structures that value filial loyalty and community duty don't give people a pat on the head for attacking their neighbors. Even if your neighbors are terrible people, you suck it up. And so what I'm getting at here is, yes radical islamic online content can make a mass shooter. And violent alt-right rhetoric can make a shooter. But I would argue that CNN, Fox, MSNBC, Law and Order + L&O:SVU, CSI and all its spinoffs, those are radicalizing materials as well. They're just not nearly as intense. But most people people don't go on some shifty ass website with bad formatting and domain name you've never heard of so that they can read about how christians/gay people/people of color are a disease. Most people do watch mainstream American TV.
*it was banned at that point but the very young are most vulnerable to lead paint, so in the 80s the affected got old enough to be dangerous