I'm 17 and in full time education at sixth form (UK). Can my employer enact disciplinary action if my shift is in school hours and I can't make it due to school?

I wasn't sure what subreddit to post this in - but my title is pretty self explanatory. In the UK I am classed as a young worker, however I am also in full time education at sixth form. I also have a part time job at KFC in order to make some money while in school. When I first joined I made it very clear that my availability was 5pm to 10pm Monday to Friday and 10am to 10pm Saturday and Sunday. There was a period right after when lockdown was enacted that I was not expected to go into work and was placed on furlough. After about 8 weeks I was expected to return, however my availability had been changed to 10am to 9.30pm Monday to Sunday without me being notified. They didn't tell me they were going to change my availability, they didn't even ask to update my availability they just straight up changed it. And of course, it was okay, I wasn't strictly "back at school" anyway so it's not like it affected my learning as I could still complete school work outside of actual work. However after June 1st my school is doing online learning within normal school hours so I would be missing out if I had to attend work instead. So... my manager posted the rota for this week and I was scheduled for... 22.5 hours... I said upon joining I wanted a maximum of 18 hours per week as to not fall behind without school. My school recommends a maximum of 8 hours per week and most places recommend a maximum of 12 hours a week. Anyway, I emailed my manager and I explained that I could not do those shifts, as I am in full time education and the shifts were in school hours. I explained how my education was vital to me. My manager only responded by telling me to "drop my shifts" and "see if someone picks them up". My concerns about availability were not addressed, and neither the ridiculous amount of hours my manager expects me to work. So I dropped tbe 2 shifts I couldn't 2 due to school, 1 of them was picked up however my tuesday one wasn't (2nd June). It says that if no one picks them up they remain my responsibility, so I'm not sure if they have become my responsibility. It says in my contract that an unauthorised absence is misconduct and can result in a disciplinary hearing and result in disciplinary action. However, I clearly stated that; a) I could not attend those shifts as I am in full time education b) my availability had been changed without my notification and that it would be unfair to assume I could work those hours c) I had given 4 days prior notice to not turning up. Now I couldn't really find anything that clears this up but is it illegal for my manager to expect me to work during school hours? Or is illegal they changed my availability without my notification, and without my consent, and expect me to work in those hours? What are my rights as an employee and can my employer take disciplinary action? Thanks for any and all help.

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