I'm not sure you will have officially enrolled yet.Some colleges will offer you a place and you can verbally accept, maybe fill in a form or two, but you don't actually fully enrol until weeks later once you have 100% firmed your subject choices and can be registered for your exams. At least, that's what it has been like in the two colleges I've been to. There is a lot of moving around in the first few weeks at sixth form for most students. Loads of people drop out and go elsewhere in the beginning, before or after enrolment. They can't force you to take your exams with them. I would say if you want to move colleges, go. They're right, you can always just not turn up or drop them an email. You have to make sure you're choosing the right college for you, you are going to be there for the next two years after all! Good luck with college!