Time restrictions - text/phone

Is there any way to place time restrictions on text messaging or phone calls on an iphone or on att's service?

I've got two teens with a texting and talking on school nights problem.

Currently I have to take the phones at bed time, but thought it might be easier to have a time restriction setup (so they don't do it during school too).

Verizon had parental controls on their website that could enable time restrictions, but I can't seem to find anything like it on AT&T.

An app or setting on the phone would be great though... any thoughts?