I am in 7th grade but I'm supposed to be in 8th grade this school year what do my grades have to be to take a test at the board of education/

Two questions:
What country?
Why do you say you were supposed to be in 8th grade?

Also, usually skipping grades is arranged between years, not after they have started.

This will vary depending on the school system. You need to talk to your guidance counselor at school.

First many school systems seldom if ever skip grades, Your parents will need to go and speak to the school system.
And when it is done, it is almost always done at the end of the year, so you have the summer to prep for those things you will miss out on.

It gets harder as you go into Jr High and High School as many of the areas of math and the such build form the year before. So not only will your current grades have to be near perfect, you will have to also study the material, that they year ahead is also learning.

They may look at the end of the year test, some systems use those like the CAT test but all use some type normally.
So you will have to be in the 90's, most likely high 90's on it, and then take the test for the next year and get a solid passing score on it.

** I have known a few students who have done this.

So why are you not in your current grade you are "suppose" to be in?