Do you have a lot of tests? Why do you expect yourself to do better than you've always done? I have a teenager in my house and deal with many teens who expect more of themselves than possible. Your capacity for learning has not changed only the subject: harder math, different novel, history subject (like ww2 verses vietnam). If you know the subject than you are bound to do better, if you don't know it than your bound to do as best as can be expected. You want better than that? Well then "want" to be interested in that topic, study it, and do your best. Your attitude when going in to a new subject is what your going to do. Take a deep breath, slow down, and think "this is like studying at home", "i read this", "i know this", and best of all "this is easy". If you think hard, it usually is. If you think school is easy, it usually is. Why? Because it's the same stuff you've done since first grade with more details. Geography is the same, we have not had a massive polar change. Put your sense of humor to work too, that helps my comic daughter of 8 yrs old.

By the way, sometimes study groups help, by you teching others, helps you to learn, and engraves topics of least interest to you in your memory. If you don't have one, start one, libraries are a great gathering place.