Contracted hours.

Hi, im an apprentice at a garage and I dont do any time in college alongside just full time in the garage but I have training courses to travel to whenever I have one coming up. The apprenticeship has a set rota for under 18 apprentices with a Sunday Monday as the days off. im 18 now and officially on the work rota with the rest of my colleagues (except the other apprentice who's 17). It works with having 2 days off in a week, not set just get what your given really, unless you ask or trade with someone. I turned 18 on the 28th of February and before I turned 18 my boss asked if I could come in the 7th which is tomorrow. im coming home from my work course now which is about 3 hours of travel and as it stands I've done 40 hours of work this week already which is the usual but after a course id have Saturday Sunday and Monday off. Im not sure why I said yes to my boss being fully honest so I mightve just done myself over as is but because im 18 im not too sure how it all works with my hours and everything because I know it differs slightly with the hours for under 18s compared to over 18s. The day tomorrow would consist of 8 and a half hours, im just writing this post to see if theres anything I can do to stand my ground because im all for being a hard worker but not when it'll get too stressful and tiring on myself. im not sure why I said yes I just felt pressured to say yes but I dont want the wool to be pulled over my eyes and get taken advantage of for my lack of knowledge in what's right and what's not. I've done some research slightly on Google but ill have to reread my contract and see what it states. Just writing this post to hear anyone's thoughts and if there is any, just some advice going forward would be appreciated. Thanks people of reddit 🙂

p.s. sounds stupid to probably everyone else 🤷🏻‍♂️😂 but I asked for a half day earlier this morning to see if I could at least get a break but he said he asked if I was okay with coming and I said yes so they've booked jobs in (the garage i work at is extremely quiet compared to the others in the company) so I can take that with a pinch of salt but still he has 3 other members of staff in tomorrow including himself so i dont understand why he would want me in either besides from the fact that it'll save him doing any work.