How do I stay motivated to do well in school?

I'm sixteen years old and next year I'll be in my sophmore year. This year, I was in all H1 classes (the highest you can be in) and I did alright-I did really well the first term, but after that my grades were mostly B's. That screwed up my chances of getting into AP Calc senior year, because it's based off of your Algebra 2 score, and I missed it by about four points. So I want to stay on track to get into good classes junior and senior year, but I'm not sure how to convince myself to buckle down. Any tips you have are welcome. Thanks!