When I joined cheerleading in the seventh grade, it was stressful because the majority of the girls were already extremely good at stunts and gymnastics while everything was new to me.

should there be a comma between gymnastics and while?

I would use instead of "while" a semi-colon and "however, everything was new to me." "stunts and gymanstics; however, everything was new to me."

I might even rephrase - " Joining cheerleading in the seventh grade was stressful because everything was new to me. The majority of the other girls were already extemely good at stunts and gymnastics."