Replying to Work Advice

So I'm working in my first adult type job. I started out as shop bitch. Now I'm still shop bitch but I also run a cnc router along with some other machines. I make $12 an hour for working about 17-20 hours a week. I'm a full time student so this is just to help pay bills. This summer I'm definitely going to need full time. So my issue here is, how do you go about asking for more hours during the summer and letting them know if they don't give them to you, you're going to leave? I don't want to come off threatening, I just can't afford to work that little all summer. I have a few other offers for the summer that need an answer asap.