It sounds like everybody has given excellent advice, and I think you're also on the right track. My only experience leading large groups of young people (college age) was for paintball, not hiking, but here's what I learned:

- Make sure you're legit. Checking up on group size limits and having a mandatory waiver (you can find generic ones online) are a good start. You can probably pick up a lot of information at a local ranger station regarding do's and don't's as well as have-to's.

- Give them structure. Spell out the plan before you start, and announce things like lunch at the right time. Communication will be an issue for hiking because large groups tend to really string out, so it would be a good idea to make sure your leaders stick to the schedule too. Don't depend on the walkie talkies - they often only function on line-of-sight, so signals may disappear depending on terrian. Also, try to pick leaders who people will listen to. People usually will follow leaders who are friendly and know a lot about what they're doing.

- If you're looking for comprehensive first aid/CPR, you might be interested in WFR training. That's Wilderness First Responder - it's in between first aid & EMT training, and is often a requirement for camp leaders.

- Be prepared for cuts & scrapes, or worse. Young reckless people who aren't used to the outdoors will often fall & get hurt, and the people with big first aid kits will become popular. Also check if anybody in the group has any special medical conditions. (My paintball club once "lost" a guy who turned out to be diabetic, and we were afraid he'd collapsed in the woods somewhere. After searching for a while, he showed up back at the cars; turns out he'd gotten lost in the woods and found a road, then hitchhiked back.)

Hmm. That about covers it. You can find lots of books on hiking for beginners, and some might be useful resources for teaching newbies.