I'm so sorry, I had a difficult boss a few years ago. Same thing - mood swings, unreasonableness, and just a difficult person to be around. I ended up quitting. I wasn't the first person to quit because of him, and I wasn't the last.

I decided my mental health and stress level was worth more than any job. I really liked my job, so it was sad, but I couldn't deal with my boss flying off the handle at really little things. The last straw wasn't really anything he did to me, I witnessed him coming down hard on a summer intern and I knew I couldn't stay employed there.

Looking back on the whole situation I would have handled it differently. I should have talked to the owner of the company or the personnel director, but I didn't do that until after I quit. It doesn't sound like you have that option, since you work for a small office.

Maybe you could discretely start looking for another job, and maybe even find something better? Stress levels at work (plus school) can do a number on your physical health. Take care.