Hey,
I am 17, working at Tesco. It will depend on your manager and other colleagues on how flexible they'll be willing to be with you. It did take my entire application process (hearing back, interview and induction) around 2 months, which wasn't ideal. However from what I understand from the store I work at is that the basic expectation is 15 hours a week contracted, with the option of overtime (if applicable). Try getting with touch with them (e.i calling up the store or visiting in person) to see if you can find out any information. My process wasn't clear, by chance I had turned up on the interview day after a some misconceptions (however, this may of been due to mangers leaving the store).
Don't be disheartened if you don't get in! All the best!