Don't believe people when they say 13 units is too big a workload!! Sure, if you're doing a lot of subjects that involve a major work, like art, design and tech, etc, it might get difficult, but if you're doing mostly theory-based subjects, it should be no problem. I have 13 units and keep up easily, it all depends you you as the individual.
Guernica - Why not keep all your subjects? 13 units isn't too bad, and you can always change your mind later if you're finding it too hard to keep up with the workload.
HarryKozy - I had the same problem as you, I was doing 2u english adv, 3u maths, chemistry, physics, music 2 and ancient history. I wanted to pick up music ext, maths ext 2 and history ext, but decided 16 units may not have worked....Even though I loved all my subjects and didn't want to drop any.
I ended up dropping An history, purely because it didn't tie in with what I want to do at uni, and picked up the maths and music extensions.
So, it all depends on what you want to do after school, if you've made up your mind. But I would suggest maybe dropping a science if you want to pick up both those extensions. UNLESS of course you want to go into a science field after school- then drop Mod history.