Part of raising self-disciplined, responsible kids is giving them opportunities to show you and themselves that they can assume more and more responsibility. If they fail to maintain a healthy school/social life balance, then they will receive an important lesson and can be given a chance to remedy the situation. State what your expectations are concerning their academic and family responsibilities. Then show your confidence in them by saying, "I'm sure you can handle this."
It's truly much more healthy for them to be able to spend more time with their peers at these ages than it is for them to feel forced to spend every night, all night, with their parents. You will know soon enough if this experiment is successful.