In the long run, the grades don't matter, but your attitude WILL matter. The girl you're "competing" with is probably someone that you'll never see again after the next 6 years. Don't let her accomplishments affect your own self-esteem. Again, ask your teachers for help. If your science teacher won't, ask your math teacher, or ask another teacher you had and respected last year.

You should be motivated by that grade to do better next term. Ask for extra help, ask if there is something you can do for extra credit to get your grade up, then work as hard as you can.

Don't get depressed upset, do get motivated upset. And you know what? Your learning is more important than your gradees anyway. When you take the SATs 4 years from now, they are not going to ask what your grade was in science in 6th grade. They will question your KNOWLEDGE of science.