It is hard to enforce a contract provision that has not been enforced. That is why unions enforce provisions that seem silly or to have no bearing - "use it or loose it" is an underlying part of contract law.
Please note that I said 'hard' - not 'impossible'. It depends on how much effort people are willing to expend to enforce it once it has almost lapsed. School boards usually have more luck in getting these things enforced after a lapse than teachers. Illinois judges tend to favor employers when dealing with contract lapses.
I hate to say it but when I was teaching I saw the same thing happen more than I care to remember. Your contract is your friend - read it, know it, live it.
