There are an almost infinite number of factors here. Like many other times in life, you will get what you pay for. I'm sure you could find a carpenter who would work for $8 an hour, and there's a pretty good chance that he MIGHT be able to tell a band saw from a radial arm saw. Trust me, you don't want this. I have no idea what the standard union rate in San Francisco is, but chances are it hovers somewhere between $20 and $35 per hour for a union carpenter. At that rate, you can probably get a pretty good carpenter to work, however keep in mind that if they're already a good carpenter, then they're probably already working somewhere, and every hour they spend working for you part-time with no benefits is one less hour they can be working at their real job or spending time with their families.
Also keep in mind that a good carpenter is not automatically going to be a good carpentry teacher. These are two totally different beasts. It's relatively easy to find a carpenter who can be handed a stack of plans and can turn out finished builds. It's quite another to find a person who can work with students to teach and instruct in order to come to a finished product. Also remember that since you're in a high school, you can't just put an ad in the paper and hire the best guy who shows up. Your situation may be different since you're at an arts-specific school, but if it's anything like most public high schools, even something simple like this will require a long, drawn-out interview process, an extensive background check, drug tests, and almost certainly bachelor's degree, if not even more.
Hope that helps.
