I'm sorry to say you're not the first and you will not be the last.
I wish I had better news but there is no such thing as "inexpensive" legal action in the USA.
You can start by having an attorney draw up a letter which will no doubt be ignored.
What this school is gambling on that you will not want to spend $15,000+ in legal fees to get $20,000 returned.
What is the stipulation in the contract the school is using?
Did you not show up for class? Cancelled lessons? Fail a check ride? Anything else to disqualify you from a refund?
Those are BS reasons to keep your money but the school will use that contract against you.
Take one of the airplanes, fly it to a different town and call the school, offer to return the plane for you money
Unless you have a lien on the airplane that is grand theft which is a felony and after your jail time you will be deported.