FWIW, there is a group that does this and other trips called SHP (
http://www.bicycletrips.com/).
You'll need more than you think for example: lawyers to write up wavers and plans for every contigency.
I'm currenty riding cross country and will tell you that 80+miles a day is too much to ask for most touring cyclists. Your asking for 8-hours a day of hard riding from kids. When you're riding accross you want to be able to stop and see whats going on, and you just can't do that pressing that hard. I don't think they'll be having fun after a few weeks. Also note that when they hit the first mountain range that 80mi average is gonna collapse and your gonna hyave to pull a number of century days to cath up.
I wouldn't look for more than 60mi averages from 16 year olds unless they're in very good shape. Forb 7th graders, I'd want below 50mi averages.
Also, you'll want to have some sort of quality control on the bikes. For cross country, things liek 36 holes wheels are a must, there are 200+mi streches without bike shops.
Speaking of that you'll need to work in wiggle room. Bikes do break, people get sick, and 20-30mphy headwinds happen in both directions (and it sucks, hard, think very long 50mi days)