first of all, make a routine, add exercise to this routine/schedule. exercise makes you feel better and is a stress reliever. when studying remove your phone and other things which could distract you, make sure you study. my teacher always says that we stress too much, because hsc isn't that...
bio scales better,m best-scaling science subject, although do what you enjoy more as it will motivate you to become better in it.b also, if you don't like the teaching style just ask to move to a class with a better teacher, or ask the teacher to change her teaching style