I keep things vague in my policy book regarding refunds of deposits. The only time it is kept for sure is if a parent does not enrol once the deposit is paid.
I basically keep it vague because it really depends on the circumstances. I do require two week notice and expect this also during the trial period. I am allowed to terminate without notice at any time and will refund and have refunded if it was a kid issue (mainly seperation problems that could not have been predicted). Basically I will temrinate and refund money to get rid of them quicker
If it is because of parental issues and not following policies etc (preventable issue) I will keep the deposit. Basically if a parent pretends to understand my policies and agrees but then is a total mess they get kicked out and I keep my deposit (has not happened yet!)