Well, I'll say based on my own experience, I'm presently in year 12 as well and I could not get into any summer schools for dentistry because I was not eligible. Its not mandatory, it would be very good if you could get in but if you can't then find other ways to make you a strong applicant, like getting involved in voluntary services, super curriculars and so on. It can be disheartening because I applied to quite a number of them but got the same reply but it does not mean you won't get in. You'll be fine, just work on your grades and other aspects of your application. Although it is a much easier route because of the reduced offer and all but do other things EPQs, essays, charity events, presentations and I wish you success ahead.