sixth forms are way more strict than colleges... if you're not in your parents will be notified. Maybe you should try to be more open about communicating with your parents especially if your 18 what can they do? They just have to face the facts about what you do in your free time. Tbh the doctors should have taken into account that you are in full time education and given you a better time.
Could you not tell your school that you have a doctor’s appointment first thing, and that you will be in after that? As a 6th Former, the school surely wouldn’t expect your parents to attend all your doctors appointments so should accept that that is where you are.
Tell your tutor that you have an emergency doctors Appointment so have to leave at break time - or ask the doctors to reschedule?