The IRS has some rules regarding dissolution of 501(c)(3) organizations; since you changed the name and perhaps have not established the new organization as a 501(c)(3) charity, it might actually be the case that he was required to donate the funds to the school.

Like Jewel said, spend your energy on something else.

Your new details don't change my advice to your original post:

Although this guy sounds like an unscrupulous scum-bucket who deserves nothing better than to have his head handed to him in small claims court, I think your organization would be far better off spending your time and energies shutting down any access he has to your mail/accounts, etc. and moving on.
Chasing after him for the tricks he pulled seems reasonable, but it will drag on forever and will drain everyone involved. Plus, the outcome isn't, no matter how much it seems it should be, guaranteed to be in your favor. Judges can do strange things when having to pass judgement against a well-connected citizen. So, just say "next!" and focus on re-building instead.
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The fun continues. Apparently the original $475 check given to the principal was lost. Another check was given and I just noticed the old PTO president is listed as giving $475. to the school, again. Only this time it is listed as a gift from him, not any parent org. It could very well be a personal gift, doubtful as the amount is the same. We, my co chair and treasurer, have a meeting with the principal this week to discuss the issue and to help us determine if we should take this further. My husband tells me I should just let this go, the money is gone and it is taking up to much of my energy. Any advice?