Greg,
It's a good idea to try and do well in all of your subjects at school- airlines are interested in general academic performance as well as performance in those subjects they specifically require for entry into the company.
The entry requirements do vary from airline to airline, but generally, at least 5 GCSE's A-C grade including Maths, English and a science subject, and 2 A Levels
preferably in a science/maths but this is not by any means essential- if like me you are hopeless at science and maths, it is better to have 2 fantastic A-Level grades in other subjects than two D's in science/maths. These A Levels do have to be in academic subjects however, so that rules out some of the more obscure ones. These are generally speaking the minimum requirements.
A University degree is not actually required, but if you are applying for sponsorship with an airline then I would recommend it- you are looking to be above everyone else to get accepted onto the scheme, and a university education may well put you above somebody who left school/college at 18.
The other thing is that airlines don't want geniuses- they want people who can work well as part of a team,who have dedication/motivation etc etc, and therefore it's a good idea to participate in some extra-curricular activities- a sport for example. If you're hopeless at sport, try joining the air cadets or doing something else on offer at your school/college. All this kind of thing is looked upon very favourably by airlines.
One last thing- presuming you are looking for recruitment in the UK, make sure you comply to the CAA's medical requirements to obtain a Class One medical certificate- their regulations are very strict, and if you don't meet their requirements, airlines won't look at you twice. A Class One certificate really is needed for an airline career. Have a look at the CAA's SRG site- afraid I don't have the URL to hand. It might also be worth having a look at BA's recruitment site at
www.britishairwaysjobs.co.uk their medical section will detail the requirements you need for BA.
Good luck! It's a difficult career to break into- you'll need determination and a real will to succeed, even when things look tricky and the eventual 'goal' appears unreachable. I'm sure that once you get there it'll all be worth it
Cheers
Landing_24R