Talk to your student and find out if he is feeling stress due to regular homework assignments, sports, involvement in clubs and organizations and extracurricular activities. If he says he does feel somewhat anxious, discuss some possibilities for finding relief, even if it means eliminating some activities.
Remember you are the adult. If you feel she has taken on too much, step up and say so then do something about it. Don't be surprised if she gives you an argument. After all, she has been told she is supposed to get involved from the time she entered school, and many times the issue is overrated.
Reassure him that it isn't necessary to be involved in so many activities in order to achieve his goals. There are many ways to go to college and sometimes too many extracurricular activities can do more harm than good, especially to his grades. Sit down together and decide which is more important; good grades and his health or all those checked boxes on a college application.
There are many ways to reduce stress. Each student is different and finding the way that works best for her requires her help. Ask her what she enjoys doing, what gives her down time, what relaxes her, and make some suggestions. Sometimes she can find relaxation by something as simple as taking the time to watch a movie, going to a yoga class or seeing a play. There are some students who find relief in exercising, paintball or a neighborhood game of football. Playing sports for stress relief may surprise you, but please realize it isn't necessarily the sport that's giving her relief, it's the exertion of pent-up frustration and the lack of pressure to win the game.
Encourage your student to do the logical stress reducers, such as keeping track of his homework in his student handbook, don't procrastinate but plan ahead, eat breakfast as well as the other main meals, get enough sleep, spread out the weekend homework rather than trying to do it all in one day, vent verbally to someone who will listen, and try to maintain an optimistic point of view.