1. student incident report
When a student does something wrong, a teacher notifies the administration and it is documented in a student incident report, or s.i.r. It's a catch-phrase for most teachers.
2. suck it repeatedly
When the teacher says "S.I.R.!" to a student, the student will reply with "S.I.R.!" too. But it means the second definition ;D
1. "Alice! S.I.R.!" Mrs. Stover said.
2. "Yeah, S.I.R. too, Mrs Stover!" Alice replied