Trespass Act 1980, ss 3-5
Yes. There are two kinds of trespass notices:
warnings to leave
warnings to stay off.
Both kinds can be given either verbally or in writing. They don’t have to follow a set form or use a set form of words.
The school principal a board member, or any staff member can ask you to leave. Other people (another parent, for example) don’t have the legal authority to do this.