pay periods

In our last contract, teacher assistants were offered 26 pay periods instead of the usual 22 (to help get through the summer). Library assistants were told by our support staff president, that they were not allowed the option "by law". My understanding of the law was that teacher assistants needed to be offered the option but that there was no law stating that we couldn't have it & that it was a negotiation issue. Anyone have info about this "law"?