I've been rejected by McDonald after two interviews, Hollywood bowl and by KFC, even though I have 5 8s and 9s and 5 7s.
Personally, I think there are three main reasons that a 16 - 18 year old isn't in a very good position when it comes to getting jobs
1) Lack of hours/flexibility
Because of your requirements at college/university, you won't be able to do that many hours, and they aren't able to just call you in when they need you. This is the reason you see college/school dropouts at the likes of McDonalds, because they can be called in whenever they're needed, whereas you might be unable to due to study commitments.
2) Overqualified
If you have good A levels or high GCSE grades, the employer will know that you will have college commitments, but also that you might think it is going to be a breeze, and something you reluctantly do to get money, which could lead to an attitude problem. Whereas a college dropout might see it as something more serious, as he/she might not have the qualifications to go on to anything better.
3) Job security
This kind of links to my last point. Due to future aspirations, the employer could be aware that you are going on to bigger things, therefore, you're probably going to leave in the next few years. Therefore, they will want someone who genuinely has a future in that company, so they might reject you on that basis.
Honestly, don't be worried. I got rejected by that McDonalds TWICE and they are still advertising THE SAME JOB. Plus, I went in there a couple weeks ago and it was all over the place in the kitchen.
So don't be disheartened if you get rejected. They're not good enough for you.