My best friend wants me to fight elementary schoolers.

Yeah.It's as strange as it sounds.We are both 15,and we have met 5 months ago and we have bonded a lot since then and we trust each other,and we both were there for each other when we had it rough.Now,my friend is a bit hotheaded and can't think well when he's angry,and he particulary argues with a group of kids in 7th-8th grade (trust me he is a good person I swear).The kids are,to say the least,abnoxious.If I ever acted like that my father definitely would have killed me.Since he is short because of his genetics it's easy for him to get picked on by those kids.He would mostly ignore it,as i advised him too.It never went far except a few glares,but I always advised him to stop it because the best way to deal with something as ridiculous as this is to ignore it. Imagine my surprise after I go home and decide to sleep,and I hear someone calling me on snap chat.He asked me if anyone was in the room w me,but I was alone then so I picked up.He told me that he,HIS LITTLE BROTHER who is the same age as those kids,and his FRIEND IN THE SECOND YEAR are going to FIGHT THE ELEMENTARY SCHOOLERS,and he requested me to join him.At first i was shocked,but then I asked him if he's pranking me.He was dead serious,and the second I realized that I started telling him not to do it.Now,I can't turn him away, because that would definitely affect our friendship,as he has always been there for me in any (but certainly not as weird) situation like this.I told him to be a bigger person and walk away from this,but he told me that he's just gonna fight them so that the "Damn threats over Instagram" stop.We hung up shortly after and I have to be there tomorrow.First off,I don't condone hitting children under my age,it never sat right with me and I found it disgusting not just because it's wrong but because I had problems like that myself with the older kids when I was.Second,I can't bring myself to do it but I can't let my friend down either.Ive done all I could to talk him out of it,but he won't listen to me.Please,give me some advice.