I'm very busy while in school, but I like it that way. I usually take about twenty credits a semester (trying to double-major in four years), TA for an animal science class, complete about 15 hours of research a week, and put in time for several clubs. And I do have friends. 😛 It's a lot to juggle, but I feel really bad being at school if I'm not busy. I don't really get stressed to the point where you cry or get the shakes or want to give up. Being "stressed" motivates me and forces me to plan to get things done.
I don't really have a system for balancing things. Everything gets done at some point, and usually sleep doesn't get sacrificed. I work before I play, and I have a pretty good idea how much time everything takes, so I know when to start projects or studying or whatever. Also, I save volunteering and working for the summer. I honestly wouldn't have time to add a job on top of everything, so I'm okay with accepting that. You need to be okay with your choices, understand your priorities, and know your limits in order to juggle your schedule.