I was also asked the same question last year before I was appointed as Deputy Head Boy for 6th form, so I just mentioned how I could improve the moral of the students by setting up a reward system for the students who always do well but never get noticed, increase the number of career events that are held for the 6th form, improve the facilities that the 6th form has to help improve the attitude of the students and to provide better equipment to help them achieve the grades they needed.
Personally I wouldn't really be able to give you some ideas unless I knew what you're particular school was like and the issues it may have, so I'm sorry I can't help you further but maybe someone else will be able to provide some assistance.