There are a couple great girls on the Plano West squad this year as Seniors.
Perhaps you could email the sponsors at the senior highs. I am sure that would count for NHS service hours as well for the older girls. I know Kelly Wonsmos is the sponsor at Plano West this year.
If you go to Pisd.edu and follow the links to the individual senior highs you can find the athletic coaches list there that will link you directly by email. I can't speak for all of the squad, but i do know a couple of girls that would be great that have very good character.
If you are closer to PESH or PSHS, you could do the same with those sponsors.
It would be worth a shot.
From a former public school educator...don't push the cool factor too much. there will be PLENTY of people to influence her once she is there. The "cool" kids are not always the ones you would want your kid to be around!!!
I am sure she will be fine. I would also check out other clubs in the school...there are a ton of opportunities to get involved in things that she is interested in with kids that have similar values to yours. You can always take her to the clinique counter at the mall and do that WITH her to build in that relationship. That way she is with you, but not taking YOUR advice, it is a 3rd party "professional". Maybe even let her play hookie from school this spring and have a mom and me day. I think Nordstroms offers free personal shoppers if you call ahead. You wouldn't really need to buy more than one outfit, not expensive, but it might be fun to give her some ideas. Then you can go to other stores and check out their stuff. I would be a little leery of an 18 year old showing my 13 year old the ropes on clothes...as there is a VAST difference between those! She will grow up fast enough on her own!
I knew several students who seemlessly made the transition from private to public schools. Key factor was a parent that was involved and had good communication with their kids! You can always go up to the school or check out the website to see what opportunities are there. If I remember correctly, the MS cheerleading squads are HUGE and not done by tryouts...so there will be a VAST range of kids there. It is more of a "spirit squad" than a cheer squad like you might think. At the HS level is where the competition of it all takes form.
Good Luck...sounds like you are doing all you can!