Schools in the US are a shit show with regards to how to handle terrible kids. Teachers can't lay a hand on them, nor can they remove bad kids from a classroom because they deserve the same education as everyone else. My mom was an elementary school teacher then counselor for many years, and she said there were kids that would throw tantrums several times a week - the rest of the class and teacher had to leave the classroom until a counselor/admins could calm the kid down, which sometimes took hours. Who knows what the backstory of this kid is, but it's possible he's a little terror and the teacher/school just got tired of dealing with it so they decided to call the cops. Also possible it was a completely unnecessary overreaction by the teacher.
Edit : just read that the kid has emotional and behavioural disabilities. He probably causes problems on a daily basis, his parents won't do anything about it, and the school simply got fed up and said screw it, let the cops deal with it.