Parental Controls/Profiles
I'm assuming this isn't possible in the current release, but from my years of experience in IT, when you join a network you start with low levels of access, then access is built up.
My eldest son has found a loophole, whereby he can adjust his MAC address on his iPhone which is pretty easy to do, so just resets this to rejoin the network outside of his alloted profile, and have wifi access through the night. I don't want to confiscate his device but I don't seen any other way to stop his continued access out of alloted hours... Any ideas appreciated.