When I say decide it's more like confirm. You apply for 5 universities further back down the line and then depending on the grades you have so far and your personal statement, they either give you an offer with a certain set of grades attached or reject you. You then choose one of the others as your "firm" offer and one as your "back-up" offer. Then you sit the exams I just did and if you manage to achieve those grades, then you secure the place and get to go to your firm offer. If however, you don't, they'll reject you and you have to see if you got the grades to go into your back-up offer instead. Fail that and you have to spend a day or two franctically ringing up all the universities that still have places hoping they'll let you take one.

Some universities will be a bit lenient with their offers and let you in if you drop a grade in one subject but most big ones won't meaning you have to really work in the May/June exams to make sure you get the grades you need.

I see; here I believe you generally take the SAT/ACTs before applying. It's a bit less structured (no 5 school limit on applications that I recall, much less a 2 school limit), which can be nice for applicants, but is probably a bit harder on places like Harvard that I'm sure get a million people spamming them "just in case" they get let in. There are fees though, maybe for applying (don't remember) and definitely for official school records to be sent to places, so you can't just send out hundreds of applications willy-nilly.

Here, having a "backup school" would just mean sending your application to a place you're sure will accept. It takes a little while for them to get back to you with a decision.

Ah right. In England we have a centralised system to deal with university applications called UCAS. You just sign up and fill in all the details and then pay the fee (can't remember what it is but it's standardised so it's the same no matter how many places you send applications too) and then everything is done through them. Offers are updated on your own personal account on the site and you choose to accept them through there. Helps take away a lot of the stress because you know there is an experienced organisation dealing with you application which you can ask questions too etc.

One of the nice things about UCAS is that you can put in the centre you are studying at and even the form you are in. This means that when you send off your application, it's in reality being sent to your teacher so they can look over it and make sure that it is as good as they think it could be. They also write references about you which are attached to the list. Again, takes away a lot of the stress because someone you (hopefully) trust will have looked over it and thus you know it's the best application you can give.