How about embroidery on jeans, shorts, or tops? The girls can embellish an article of clothing they currently wear.
I can think of several sewing projects:
Pillowcases, hand bags, sachets.
You could do projects out of recycled jeans:
http://sewingneedlework.suite101.com/article.cfm/recycle_and_repurpose_old_jeans
Working with tweens and teens can be a ton of fun. But organization is the key. The first meeting I would go over safety issues, basic machine usage, and decide on the project. I would also have a sample of the project(s) ready for the gals to see. Many times these gals have not been exposed to sewing machines or any needlearts.
A couple of things to think about:
are all the girls going to work on the same project? If not...you could be doing 8-10 different projects during your time.
what is your budget for each project?
Be prepared to listen to some 'interesting' music during your sewing time!! LOL
FW