I'd honestly think that the problem lies within year 7-10 itself. People tend to not choose maths and science in year 11/12 due to poor teaching practices and general disinterest in the topic - probably since we have crappy teachers teaching such subjects. We are generally exposed to lots of content, rote-learning and memorizing in Science. Mathematics is also pretty easy in itself but it's bloody boringly taught - people don't really see the rewards and get easily bored. Then when you get to year 11, BOOM and then people start fucking up due to the huge change in difficulty level. Teachers play a big part in this, as well as the syllabus itself, and they gotta change the syllabus, as well as hire better teachers to account for this.
As much as I'd like maths and science to be compulsory in year 11/12, there has to be significant restructuring of the syllabus - if anything using the framework of the IB would be a good initial start, provided that the syllabus in year 7-10 accounts for the change in difficulty for year 11/12. Bring back the mathematics involved in the physics syllabus.
Also I believe languages should be compulsory for up until year 10 - having a 2nd language handy can actually help us to be more competitive and capable individuals in the future. The few problems with this is about managing the quality of the teachers teaching such languages, especially in the countryside. :S