McDs rely on young workers because they can pay you so much less than the over 22year olds, and because you are often new to work and feel obliged to take all the shifts that come you way.
Don't quit. But when you go in next ask to speak to your supervisor and tell them that your availability has changed. Tell them that over Christmas and from the new term onwards, you have more study work and can need to change to 2 shifts a week.
Dont feel guilty, dont feel apologetic, dont feel like a failure, just be straightforward and confident in standing up for yourself. They wont sack you, they need you too much but it will force them to find another teenager to take up the slack.