Would it be worth doing courses in LC maths while in ty? If yes does anyone know any good courses
Yes, It would benefit a lot. Well the institute courses are good but you may have some centre in your local area, you can join there.
It's nice to know everything before the teacher does it in 5th year, so if you have any questions you can ask her then.
If you don't get any course you can get the textbook and do questions from it. If you don't get a question post it on studyclix, someone will answer it.