All of my answer below come from my own experience. Each Squadron is different and acts in its own way, so take advice from all who will reply to you.
In terms of missing out on opportunities the only thing that will hinder you here is that there are certain parts of training you need to have done before you can do certain activities. A lot of activities require the First Class Classification and you missing that classification training may well have an impact on your eligibility.
Saying that however, you are joining at a slightly older age which if anything should work In your favor in regards to the above. Talk to your OC about your concerns and they hopefully will not have a problem with you self studying the First Class Syllabus. You will still have to pass some tests and tick some boxes but hopefully your OC will be able to come with a plan to accommodate you.
Again with promotion to Cpl before 18, because you are that little older one would hope you are more mature then those 12 and 13 year olds you may have joined up with. Take in all the training, be mature and ask for advice from the Cadet NCO’s. There is also many threads on what the criteria are for promotion to Cpl on this website so please take a look. Work hard and I can’t see there being any reason why you wouldn’t be able to make it to Cpl before 18.
When I did my GCSE’s oh so many moons ago I only missed a parade night when I had an exam the next day. This was a personal choice however and everyone is different. Take as much time as you need to do your exams as your education must come first. I cannot stress this point enough as believe it or not, one day a job will become more important then cadets. I unfortunately did not realise this until after my GCSE’s and I reckon I could have gotten better grades if I had spent less of my time with the cadets in my GCSE years.
Like I say, take advice from all who reply as every Sqn in the Corps is different. I am on the move so can only give you some brief advice!
Welcome to the Corps! Work hard both in the Cadets and at School! All the best for a great cadet career!