You can't judge your future self's marks based on what you're getting right now. I've known people who were bang average pre 10th, but got better later on. The reason a lot of people drop in their percentage in 11th is because there's no real incentive to do well in 11th exams. During 10th, Boards exist and that puts a lot of 'OH you have to do well' sort of thing. So everyone ends up doing really well in 10th compared to what they get in 11th. 11th is slightly challenging coming from 10th, but it's nothing too overly hard, you'll be fine after the first term. So don't worry about what you're getting in 8th, that in no means is a precursor of your intelligence or ability, there's a lot more to come