Only do it if you love the subject.
Also, it's so much easier if it's a theory based subject.
I started year 11 with Music 2 by distance ed. By the start of the second term, I met my teacher for the first time [who also happened to be the head of the distance ed centre I go through] and she more or less told me I should drop it.
I didn't do it because Music 2 is a partially practical subject, and it's just difficult to do if your opinion of it is only mediocre.
However, I now do distance Legal, and love it.
Language is harder to do through distance ed, I've got a friend who did Japanese this year, but she dropped it for her HSC.
Science, you'd have to find time to do the mandatory pracs, but if you wanted to do it, the theory aspects shouldn't be too hard, and the more you like the subject, the less likely you are to use your study periods for a bludge.
Alllso, study periods... Stay away from your friends in these D: