Feel like I’m not following through

I’m 16 and work at a bar in a tourist town during summers. I told my boss my last day but did offer to work weekends through September if needed. I ended up working Labor Day weekend and now I just got a text asking if I could work tomorrow. Im feeling pretty conflicted. My boss does this a lot where he will ask you to work with little notice, usually night before or morning of. I had made plans tomorrow to practice for my driving test which is coming up and do some homework that I need done, and tomorrow is really the only day this weekend I can do those things. I texted back and said I was unable to work during the day tomorrow but did offer to work tomorrow night shift (6-10) if needed. On one hand, I hate feeling like I cannot make other plans because I need to be available, but also maybe I shouldn’t offer to be available and then not follow through. I have already put in a lot for this job, when people ask me to cover I will say yes with the exception of a few times I couldn’t, I have worked when my boss asked me to many times by rearranging my schedule and canceling plans. I just don’t know how to go about this, should I have said yes to working since I did previously offer? I geuss what I’m feeling is I want to have time for myself especially now after school starting, but feel like I shouldn’t offer to work if I know I may not be able to every time when asked, something I should have thought about more beforehand. Anyways I’d really appreciate some insight here especially if you’ve been in the same situation. Thanks!