It sounds like they are asking you to work hours that you were paid for but did not work, is this what you are describing?
This answer does not constitute legal advice and you should contact an attorney to confirm or research further any statements made in this answer. Any statements of fact or law I have made in this answer pertain solely to New York State and should not be relied upon in any way in any other jurisdiction.
Just a hunch, but I'll bet that you are a member of a union or a member of a bargaining unit, and that the "contract" is a collective bargaining agreement between the union and the school district. If that's the case, one way to frame this is a dispute over interpretation of the terms of the collective bargaining agreement, which probably requires members of the bargaining unit to work a defined number of days each year and regulates how sick days and personal days may be used. So I suspect this is a matter covered by the grievance and arbitration provisions of the collective bargaining agreement--typically disputes over interpretation of a CBA are subject to the grievance dispute resolution procedures set forth in the CBA.
Consult New York counsel if you need legal advice. What I wrote above is not legal advice; it's just my two cents. I don't practice law in New York or hold New York licensure. I practice in Vermont ONLY.
If you were going to be paid but nothave to work the day after Memorial Day, and now are going to have to work and be paid that day, that would be legal. Speak to the union if you disagree.