How much should an average 8th grader be able to power clean
There is no such thing as an average 8th grader. In the "wimpy kid" books, the main character complains that 8th grade is a mish-mash of kids at all stages of development from scrawny child to young adult who needs to shave.
Initially, an 8th grader is not required to do power clean given that they are not fully grown. Otherwise, one should be able to do light 115X10 or that's light enough 135X10.