I'm worried about my grade in science?

Before I start let me say I am in grade 7 and I have very high expectations for myself. Please don't say any answers like "That's really good," etc. I do very good in English and Pre-Algebra - (A+) and I do pretty good in my social studies class (A) However, I am doing bad in science. I have a B+ in this subject and I really want it to go up. I study for 2 hours or more per test (Not cramming, but over a week or so) I get all the homework right, get 100's on all the in-class assignments, but on the tests I get a B or worse! It is horrible. So, how do I study for science? I need to know because I have to get an 100 on my next test. (If it helps at all, the next test is on Physics) Also, another question: Is it wrong to set such high standards for myself? Thanks in Advance!