My school contract says I must give 3 months notice to leave. Is this a legal amount of time? Normally its 3 weeks right?

Is there a Thai law that may over ride this contract time?

If there is a Thai law it will probably only stipulate the minimum time required.

Your school contract is what is legally binding for you.

opalhort

A contract is a contract.

However if you do leave without notice I cannot see they can do much about it.

On the flip side, if they make you leave early then you are entitled to 3 months salary in lieu of notice.