Beg for extra credit and then DO IT! It helps.

I'd ask for a tutor. I know you might be embarrassed, but in the end, it's also worth it. If it's through the school, it also may be free. You could also ask a friend you trust to help you study, or ask teachers for extra help during lunch/studyhall/after school.

I just remembered this: about two summers ago, my best friend was struggling in math, so her parents hired her a math tutor during the summer. It really helped her, and now she's one of the best math students in class!

DON'T be afraid to ask for help. You may feel bad thinking about this, but your teacher and your parents will be glad that you are doing something to try and fix your mistakes. Ask your teacher how to improve your grades, and ask for extra practices. It may seem tiring at first, but it WILL pay off at the end. Trust me. It's happened to me before, and doing extra has made me become a smarter, hard-working student. Also, learn from your mistakes. Is it how you're studying that has gotten you the grades? Be sure to throughly read and study everything before taking a test. Find out, and learn how to fix it. Also, when you learn new things, study it right away. Don't wait for a test to be announced for tests that will only give you so much time to study for. You will never know- a test may come up unexpectedly or maybe even a pop quiz. Study previous quizzes and tests to get the feel of how your teacher tests you on certain subjects, and then you know how to study for your tests. Textbooks sometimes have practice quizzes. You always have to be prepared. Staying confident helps, too. Never think that you can't do something- always think that you CAN. Good luck! 

Make sure to hand in all completed assignments and if you are not sure if there is anything you missed ask your teacher for a list. Take time during study hall (if you have one) to talk to your teachers about your grades. Ask to redo papers or tests you didn't do so well on. Study and do homework consistently so your parents can see that you are trying.

Honestly I don't see a problem with those grades; they are still passing and most are above average, but I hope this helps anyways. 

If you're unsure about a question, ask a parent or teacher for help.  If you don't understand something on a homework, then you probably won't understand it on a test.  Even just doing homework with a friend can be helpful, because you can check each other's work so you don't make a silly mistake.  That is especially true in math, where one wrong number can mess everything up!

Find a time to talk to your teacher. Seriously- teachers respect a student who can go in to talk about thier mistakes. Ask them about...

-extra credit opportunites

-what you need to work on

-resources they recommend to help you

-ANY questions you have

-what they can show you about your mistakes 

-anything you can redo for a better grade

And STUDY as much as you can. Save every paper and look at every mistake.

Hope this helps!