ISJ Rules are based on those used by the EFA (which by extension are FEI rules).
ISJ's height classes are as defined in the Entry Form. No grading cards are required. The school equestrian co-ordinator is responsible for ensuring that their school's students enter at the appropriate ISJ Level - which is why the co-ordinator needs to sign each Entry Form their school. In the interests of safety our Officials will intervene should they consider any combination to be over-faced at ISJ events.