Ask the principal what evidence is needed as proof of readiness.
In California, there are STAR test release questions.  If your child can pass that, I would say that would be good evidence (it worked for me).  Find out what the end of year test for your state is and see if there are release questions.
I do not find it acceptable to have my child be an unpaid tutor to "bring up the slower ones."  There is a difference between peer coaching and teamwork and tutoring.  Find out which is going on - no tutoring, yes teamwork.