Very interesting issue. You may very well have a valid claim against the school and the principal. However, if it has been more than 180 days since you left the school, then you will be procedurally blocked from filing a claim with the EEOC in view of the ADA violations that were committed by this principal.
That said, since you will be asserting a claim on behalf of an individual that has a known handicap that has been discriminated against, there may be other options, even if 180 days have elapsed since your departure or the last discriminatory action.
You can by-pass the EEOC and go directly to the US Department of Justice to file a complaint to let them know what has transpired here. While the actions of the principal are not necessarily a violation of the False Claims Act, you could and would arguably be a whistleblower and this would be similar to a Qui tam suit...or suit by a private citizen with direct knowledge of governmental wrong doing.
The suit could work to clear your record and result in damages for you in view of the wrongful termination. The Department of Justice could actually even file suit on your behalf in cases such as this one on occasion.
So, yes, you do appear to have a cause of action. Yes, it could be filed through the EEOC if 180 days have no elapsed since your termination or removal. Even then, you may be able to bring suit in federal court as a whisteblower of sorts that lost her job as a result of violation of federally protected civil rights of another.
Please rate my answer positively and let me know if you have any other questions or comments.
Best wishes going forward!