Clone, you need to STICK IT TO THE MAN!! I've jut always wanted to say that.
Give it to them straight. Though, I do disagree with the views on working at McD's (because some very wealthy store owners had to work there a year before they could own it), it really is just McDonald's, and it's your first job. I made the mistake of NOT speaking up when I was in the exact same situation. My parents told me to cut back, but I refused (45 hours a week). I lamed out the last two years of high school with a 3.7, sleeping through my first couple classes. I had a 4.0 honors before that. Trust me, this could be one decision that could make an unseen impact on your life.