To phone DS' school regarding the behaviour of one of his classmates.

[TheScottishPlay]

Every day DS (13) comes in from school and is asked how his day went, his answer usually centres around the disruptive behaviour of a fellow pupil. Most of the teachers' time seems to be spent dealing with this child and comforting the unfortunate victims.
On Friday DS' pencil case was trashed and a (thankfully unlit) roll up cigarette pushed down his shirt!
He (and I) have had it.
I will not be the first parent to contact the school about this.

My gosh OP that sounds awful! I'm so sorry. Absolutely - definitely let your son's school know.

Contact school and tell them you wanted them separated in class. We often get emails asking X to be sat away from Y.

Don't quite see where the AIBU comes in.

Of course YANBU.

I'd go in or write though. Things get lost on the phone.