I don’t know about UCs or CCs specifically, but it’s certainly possible to take a leave of absence in the middle of college. I think it depends on the school in terms of how long you can be away for before your credits cancel out and you have to start over. Someone I’m in college with now completed her first year, then took a year off to do Americorps, and then returned to complete the next 3 years. Someone else I’m in college with now completed her first year, took a year and a half off to (I think) work on a political campaign and do some environmental policy activism, and has been re-admitted this semester.