I was in a very similar situation at your age. I was in 7th grade, doing 5th grade math in an un-accredited Christian school (this was before the days of homeschooling). Well, I transferred to a public school, thinking I'd be way behind. The new principal thought so too - and told my Mom he might have to set me back a grade or two after testing me. A month into school, Mom called to see when that test was going to be, and he replied - "She's at the top of her class! We're not going to bother with testing!" ; )
So, don't worry too much about being "behind" if you do "get" what you're working on.
On the other hand, the secret to better performance (at nearly anything) is more TIME spent on that subject. If you currently do math for 1/2 hour a day, move it up to an hour a day and your performance will *TAKE OFF!*
And, my Mom was a math teacher, and she always said that being *very * good at basic calculation was a huge secret to math excellence. In that spirit, sign on to "
xtra math" every day - go back to addition - and do those facts (addition, subtraction, multiplication & division) till they say you're done. It only takes 5-10 minutes a day, and it will help you tremendously.
If you work on it, you WILL get better! The fact that you're on here, looking for answers, indicates that you have it in you to succeed!