Someone running away seems like it'd be the most common reason to tackle, but if stopping them without needing to resort to tackling is available (NOT GUNS! Using a taser is probably not a good idea), then it should be used.
@MSH: Did they ever teach you how to stop and restrain someone running away in martial arts?
I practiced a last resort type of technique for most of my early life which is pretty much guaranteed to keep you safe as long as the other person doesn't have like... a gun, but as it is a martial secret I must put it in spoilers which only those sufficiently prepared to follow certain paths should click, and as this can be a very intense and draining technique it is best used after dedicating years to it:
...running the fuck away, that is also the only part.
Though I would say that in the hypothetical where a tackle was being applied and resulted in injuries, my technique would look kinda silly.
I mean, you could just stand there and get the same effect instead of fleeing in the opposite direction.
Is the kid currently trying to kill someone, or just succesfully attacked someone before running away(hopefully not with scissors!), or expressed interest in suicide at a different location?
If not then what is the worst outcome of them getting away, I mean, I personally bolted out of school when they tried to escort me to detention because I laughed and said fuck off to the idea of remaining after the bell.
The next day they tried again but this time they had another teacher pre-emptively blocking off the nearby exit so I just sprinted down to the little courtyard and hopped over the barbed wire fence they needed to put in because there was a little space between two of the buildings that should have enclosed it.
Afterwards they resorted to putting me in some in-class suspension shit and tried to restrain me from leaving after the bell, calmly telling someone "do
not touch me" when you both know they aren't allowed to touch you (what, did they give me a handbook of rules and expect me to not read it?) is a surprisingly effective method by itself, and ultimately they gave up on trying to make me stick around after classes. I mean, they already had to threaten siccing truant officers on my mother to make sure I even attended during school hours, once I explained that there was no way I would quietly go along with them overstepping their bounds like that they dropped it.
Probably wouldn't get the same results now rather than what... 25... 26 years ago, but students aren't property, much less that of the school; they don't have arbitrary rights to exert control over all aspects of student behavior, especially not through violence, they're civilians too.