If Exum doesn't jump straight to the NBA, he's likely one and done anyway and would be gone by the time Simmons hits campus.
Is Exum playing HS ball in the US? If so, can he forego college and enter the NBA draft? Ultimately, unless he really wants to play colllege ball, I don't see anyway (if eligible) he doesn't enter the NBA draft. In a 2014 class that looks to be loaded, he's always listed in mocks as a top 5 - 10 pick.