yeah i felt exactly how you felf for the first few months of school, i did law, maths, bio and economics, man i hated my law lessons with a passion, so what i did was talked to my head of 6th form and i asked her if it was ok if i attended the chemistry lessons as i wanted to do that, and possible take it next year, she said taht i had to ask the teacher rather than her, and then he said that i can join and then he said that i should be entered for the exam in january, he said taht he doesnt expect much of a result as i only joined the class 2 weeks ago but he said that the expierence of a real exam would be good. so ask if you can join the class and exaggerate if by saying taht you will sit anywhere and there bound to be a space in the classroom somewhere!
good luck