[qq]There is a such a thing as giving people a second chance and warning them and making suggestions.[/qq]Indeed. If the school's got a uniform code, then that's up to them; but people should be given a chance to mend their behaviour before the knee-jerk reactionary punishments get handed down. If they ask her to change and she does, then there's no problem, and no need to get heavy-handed either.

Dress regulations can be very vague sometimes - "students may wear nothing deemed inappropriate by staff" and the like, so it's a bit excessive to come down heavily on someone who likely had no idea of exactly what was considered "appropriate".

However, if they told her off but she refused to do anything about it, then the school's within its rights and rules to start taking action.