What does it mean to repeat a cl?

So currently I have a D+ in my Honors Science cl, and I just MIGHT be able to bring it up to a C-, which is ping, by the 17th which is when all assigments are due. If I'm not able to, D+ isn't passing. I asked an earlier question about this, and looked up other questions related, and people said I'd have to repeat the class next year. What does that mean? that i'd have all my classes with 10th graders, except one with 9th graders? And I'd always be one class behind? Currently I go to a junior high, so the highest grade is 9th grade, and then next year the high school is 10-12th, so what I'd have to go down to the junior high for first period..? Please explain! Is there any way I wouldn't need to? Even if that means summer school.. or if there's any way I could avoid either? Please don't be mean.. any advice welcome!