In theory, you may have a chance.

However, the cost of doing it is probably more than $800. You need to demonstrate that Montessori did not lose any profit by your kid's not showing up. In other words, you need to demonstrate (1) Montessori can legally take only XXX number of students each year in that particular class and (2) Montessori accepted another student after you notified them that your kid is not showing up. If you can demonstrate the above, Montessori has to give money back to you. You have a common law claim if the above is true because Montessori, by accepting another student to replace your kid, did not lose anything. If they keep the deposit while making all the profits they can possibly make legally, keeping that deposit is unconscionable --- the basis of your lawsuit.