I don't believe in evolution
Neither does my biology teacher
But this isn't conventional evolution, it's people becoming crappier people
While we might be broadening the traits are speading to more people in the populace
Not to sound horribly confrontative (I actually want to know an answer here), but what does your biology teacher think of experiments where they've replicated the effects of evolution in the lab through selective pressures (i.e. things like this, where they took two decades and over 44,000 generations of bacteria forced with selective pressures and watched them "evolve").
(As I said I'm not trying to be confrontational, I'm just interested in knowing if they are unaware or if they have some other explanation for the results).
And yeah, technically "devolution" would just be "evolution" working in the opposite direction as we normally think of it. Sort of how "deceleration" is really just "acceleration" in the negative direction.
It doesn't even make sense as a concept. Look at internal parasites, say, a tapeworm - it is far simpler than non-parasitic worms of the same group, with basically no digestive system to speak of - it's essentially a fancy tube of flesh. So, does that mean that a tapeworm is 'less evolved' or that it 'evolved backwards'? Nope, it means it has successfully adapted to the environment he is living in - all those protein synthesized and cellular divisions that would be required to make an entirely useless digestive system - since the host does the job for them - is avoided.
If a change increases the odds of successful reproduction, statistically, the frequency of the change in population increases - no matter what kind of change it is. Simplification and complication are both desirable outcomes in different circumstances. And all evolution cares about is if you can survive and reproduce, NOW, HERE.
The development of medicine does not mean we're 'losing' the ability to evolve in any degree - it means we have transformed the environment we live in. If we lived in an environment where, for some reason, peanuts were the only source of nutrition ever, the frequency of the peanut allergy gene would drop quickly - for the very reason you mentioned, people dying of the allergy. If you lived your whole life on an island without even hearing of the existence of peanuts, having the gene for the allergy would not affect you in the slightest.
If anything, the example here is evolution - because genetic diversity in population is *always* a good thing - the more differences you have, the lesser the chance that one single factor will wipe the entire population out. If aliens modified all the peanuts in the world to suddenly be toxic to kill all humans, those who don't eat them because of the allergy would survive - silly example, but it illustrates the point.
That's how it worked with sickle cell anemia - a bunch of people had a mutation that messed up their red blood cells, making them anemic - a straight-up negative trait - but suddenly, malaria shows up, and because their blood cells are non-standard, they are immune to malaria. So while half the non-anemic part of the village dies of malaria, Joe Sickly suddenly can get bitten by ALL THE MOSQUITOES and take it like a champ.