From a question on www.benefitslink.com, here was the answer on whether 403(b) contributions could be mandatory:
Answer 1: "Mandatory employee contributions (i.e., as a "condition of employment") to 403bs are not uncommon. However, if the plan is not subject to ERISA, the plan would be subject to state laws, in which case you would need to check the laws of the state governing wage payment and collection, which might prohibit involuntary contributions. "
Response 1: "State wage payment laws are not an issue in public plans because the terms of mandatory employee contributions are negotiated in collective bargaining agreements between the employer and the union representing the employees...Separately if employee contributions are a condition of employment then state wage payment laws will not apply because the employee will be terminated if a salary reduction agreement is not in effect. "
Response 2: "... and even when there is no collective bargaining."
My husband at one time was part of a mandatory 403(b), but wasn't part of a teacher union at a public instituation and he wasn't making much either. But that money that comes from her paycheck is hers and she will get it when she terminates employment.

