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Dear San Francisco, California:
Given the relatively small amount of money involved, you could file a Small Claims Action against the school that refuses to pay you. Here is the information: http://www.courtinfo.ca.gov/selfhelp/smallclaims/. The statute of limitations (time in which you must file an action) for breach of a written contract is 4 years in California. However, businesses - even schools - have a way of closing, and people move on to other employers, so you probably don't want to wait that long.
You could try warning this school that you will initiate a small claims action if they do not pay your bill.