There's plenty of youtube videos that teach every bit of maths and chem. I've personally found them way useful than my teachers and I only go to my teachers if Im struggling with a specific question. If you want that A/A* you need to put some effort in yourself outside of school and improve on yourself. Yes bad teachers can impact your performance but there are several ways where you can improve on yourself without a teacher such as using online educational resources. Maths and Chem is not a subject where you just learn and revise within your school hours, you should realistically be revising 4+ hours of each subject each week outside of your school timetable.