To be fair, your assumption that all school kids are ragers is completely unfunded. You're giving way less credit than we deserve. It's like saying all black people are gangsters, it's downright discriminatory
EDIT: Since there seems to be a great deal of distaste for my comment, I'm going or clarify. By no means am I saying that so-called school kids are mature, on the contrary, they are still very much learning how they are and what is appropriate and what isn't. But this is IRL, and online is a completely different story.
Due to the fact that there only exists an online text chat on LoL, many people lose the association they hold with themselves and behind to act quite differently online, as has been found not only by researchers, but also by Riots behavior team, more specifically elaborated upon by Lyte when discussing about the possibility of an in game voice chat.
This means that while there probably are a great deal of school kids playing this game, the stereotype that the OP lists and reinforces throughout their post and this thread is purely that, a stereotype. There is no data provided, and just because, in the OP's experience, that games are generally more "toxic", could easily be due to a number of reasons, including the placebo effect, general increase of alcohol consumption in the summer, and more people taking time off work, which, mind you would not be school kids. So while some of these people probably are school kids, what irritated me the most about the OP is the fact that there is no clear concession, and that the entirety of the post is based on one clearly stated stereotype which can not be proven to apply to a large portion of online gaming communities with no voice chat.
Just because something is the case in one game, does not mean that it is the case everywhere, as conditions range from game to game. So, examples which people have used such of CoD, which is a vastly different in the experience from LoL simply do not apply in extent to which is assumed. Each game has its own "gaming culture", and people adapt to it in different ways. This does not necessarily mean that they belong to a certain subset of the human population.
Tl;Dr: My comment seems to be kissing people off, so I just wanted to tell y'all that correlation doesn't always mean causation. My comment was not well thought out, so what was listed above in the edit is more along the lines of what I wanted to say.