Japanese Continuers you say? Well, it's a pretty decent course. If you love the subject, then great! Much more emphasis is placed on vocabulary and kanji (that's what I felt anyways).
I would not recommend taking up Japanese Extension. I don't do it, but from what my Japanese teacher has been saying, it is a particularly difficult course. Unless you have been to Japan or watch Japanese television/read manga pretty religiously, it will be hard.
For me, the sciences really depend on the type of teacher and who's teaching. Chemistry does not require you to remember the elements individually, although you will begin to start remembering certain details about them after using them a lot (Hydrogen has a molar mass of 1.008
). You have a periodic table at all times.
Besides, if you do pick the wrong subjects, just try and change them for other ones. You never know. Maybe Legal Studies turns out to be absolutely atrocious or you never realised how much you hate Economics. Do it within the 1st term though or you would miss the chance.