Here’s the situation: I just finished my second co-op term (basically an internship) with my employer and am going back to school. They’re asking me if I want to work there part-time during my school semester. I am hesitant to accept as I will be having a very busy semester (doing research and also taking some of the hardest courses in our curriculum). What I want to say to my boss is “I may be down, but I will have a busy semester and will need a probably-unreasonable pay increase in order to make this worthwhile for me”

I generally lack some tact in these kinds of situations and wonder if there’s a better way to frame what I’m looking for here.
Furthermore, if he can’t offer me what I’m looking for, how can I turn him down in a way that doesn’t burn any bridges / make me look like I’m turning up my nose?