I thought that the rule about leaving a ring or stud inside the ear for a certain period so that:
a/ The hole stays open
b/ Prevents infection (provided that the ring or stud isn't made from cheap materials)
Private schools often enforce more conservative dress codes. Although even at public schools you can't wear large earrings - many schools enforce studs or sleepers. This is done for safety because earrings can get caught on stuff (e.g. during sport, practical lesson (esp. industrial arts) etc.).
Yeah, pretty much. If you don't like your school's rules, change school. But so long as you choose to go there then you need to follow their rules.