Just an FYI - the FMLA does allow an employer to "claw back" any health insurance premiums they paid (both the employer & employee portion) while an employee was on a leave, if the employee chooses not to return at the end of the leave. IF the employee does not pay them back, the employer can retroactively cancel the insurance, and the insurance can take back the money they paid for any medical treatment during leave - like the payment to the hospital for delivery - which leaves the full cost of any medical treatment for the employee to pay.
Not a lot of employers actually do this. But... with the potential risk of that and the potential loss of her teaching license for breaking a contract, I would be hesitant to risk it... She may not be planning on going back to work ever, but life happens - you could be severely injured, disabled, die, etc, etc, and she may be supporting your family.
Honesty may be your best option. Also, many school districts offer a 1 year sabbatical to new parents. I would suggest asking for that, and if they say no, then be upfront and not take a contract she doesn't plan to complete.