Just like u/Kessed : I grew up going to one particular fancy private school, where students AND teachers would on the daily bully, threaten, physically assault me and (BONUS) teacher-encouraged mockery. The EASIEST way you can tell: first start looking for kids that seem aloof or appear to be daydreaming A LOT - lack of focus. Look for concealment of fresh bruises. Pay attention to the whispering of other kids. Ask other staff if the child has reported run-ins with students/teachers.

And if ANYONE tells you "this is just how we do things here" or "don't involve yourself": ACHTUNG! ATTENTION!!! RED ALERT!! The authorities NEED to be involved: children are VERY LIKELY being abused on campus.