The number of schools receiving legal threats from parents is now so high Crown Law is drafting legal advice for school boards.

Parents have resorted to litigation after their children were cut from school sports teams and not getting a part in school plays.

LISTEN: Alex Witten-Hannah: Legal action against schools

LISTEN: Sandy Pasley: Backing up schools' decisions

School disciplinary decisions can be challenged, but Otago University law lecturer Marcelo Rodriguez-Ferrere said other decisions are much harder to challenge.

"Other decisions ... selections for sports teams, drama clubs and so on and so forth - they're not subject to the same statutory provisions."

Dr Rodriguez-Ferrere believes schools need to be subject to judicial oversight.

"Schools are exercising public power just like other government departments and we should make school official educators subject to the same regime."

The Secondary Principals Association are reporting that increased legal action taken by angry parents is becoming a very difficult environment for schools to operate.

Association president Sandy Pasley said schools wants parents' support to ensure their children learn from the consequences of their actions.

"When parents actually get into the action and try and defend their child at all costs, even when perhaps they know in their hearts their child is not right, I think that can be quite dangerous for the children when they don't really understand then where the boundaries are."

Prime Minster John Key believes schools should be able to set their own rules and have students obey them. He said it's becoming increasingly common in issues that are relatively modest such as length of student's hair.

He told Newstalk ZB having to obey basic rules helps prepare young people for the real world.

"There are lessons you have to learn and things you have to do and sometimes you just have to obey the rules. If the boss says you're to come to work at 8 in the morning, you come to work at 8 in the morning. If you're not going to obey the school rules and you're not going to obey the work rules then where does that leave us?