It might depend on the school system. I've been in a small system where there wouldn't have been a problem with it, but I've seen others where the 8th graders would be...shall we say, a bad influence?
On the other hand, if it were handled correctly, they might rise to the occassion and be good role models for the smaller children.
I'm just commenting based on gut instinct; I obviously haven't done any research on the matter.
The only “influence” on a lower grade I can remember was when one of my classmates was left back due to illness. (Back then, being left back was perjorative) The word was passed to the succeeding class that anyone ‘picking on’ this student because he was left back would have to answer to the ‘big kids.’ Elementary school students for the most part hung out with their peers. There was almost no interaction between grades.
Of course this was 50+ yrs. ago, so.................