Common Core cannot be described as voluntary. Since CC is so costly to the states (estimated $15 billion for retraining teachers and purchase of computers for all kids to take the tests), CC is foisted on the locals by a combination of bribes, federal handouts, and as the price for getting a waiver to exempt a state from other obnoxious mandates such as No Child Left Behind. Don't be under any illusion that Common Core will make kids smarter. The Common Core academic level is lower than what many states use now.
Listen to the radio commentary here: