I'd allow pupils to start specialising and drop subjects that they don't want to study or aren't likely to be useful to them slightly earlier on. In my case, I think the two hour Art lesson and two hour Design and Technology lesson I had to be in every week, not to mention the homework they gave, was quite a waste of time. I could have used that time to advance myself in Maths or Science or something that had the potential to be relevant to my future aspirations.
I think the education system also needs to be updated so as to provide pupils with skills that are useful in the modern world. For example, in our IT lessons I think we could have been taught things like programming, web design, how to create a mobile app etc. a little more seriously. In our Music lessons, we could have learned about more of the popular, modern genres that lots of people actually listen to. It would have been nice if, in our English lessons, we could have analysed what made Harry Potter and Twilight so successful, rather than just Shakespeare. It seems to me that school education tends to lag quite a bit behind real life, and that there are so many modern career paths and aspirations people might have that school gives them absolutely no preparation for.
I'd also put a greater emphasis on learning foreign languages much earlier on, because that's when they are most easily learnt (speaking and listening at least). Just as is the case in non-English speaking countries where it is compulsory to learn English, here I think we could quite easily make most children grow up to be bilingual.