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?

EDIT: extremely flexible hours are already a given.

EDIT: Alternatively, how would I approach saying "If there's an emergency and you really need me, contact me and I can do some work in the short-term for an inflated rate, but I can't afford to put in consistent hours"

Obviously, I'm aware that the "inflated rate" portion of this quote isn't a thing I should verbalize.