"He didn't knock." (spoilers)
It's pretty messed up that the principal sent Emilio away just because he didn't knock. He could have just told Emilio to knock next time. He had no right to send him away, assuming what he had to say wasn't important. You can teach someone good manners without being rude yourself. Of course, it's especially terrible considering what happened to Emilio.
I know this is based on a true story, but I hope that's one of the (many) parts that the writers made up and didn't actually happen.