Can an employer withhold a final paycheck upon termination?

Yes, you are. The employer may NOT withold your paycheck because they want their teaching supplies back. The two issues--you being paid what you are owed; and you returning supplies--are totally separate, and they need to pay your final paycheck even if you never return those supplies. (Of course, if you don't, they would then have the right to sue you for their value.)

This is illegal.  Your employer does not have a right to withhold your paycheck, even if you did need to return teaching supplies. Unless you specifically agreed to an offset for the cost of any unturned supplies against your paycheck, you are legally entitled to be paid.

Absent such specific agreement, the return of the supplies and the final disbursement of your paycheck are to entirely separate matters.  If your employer believes that you are responsible for the return of the supplies or for the reimbursement of the cost for same, their proper remedy would be to sue you for the amount owed; however, they still would need to pay you your salary or wages.