T5720 Two-Way Radios
So, maybe a purchase of four T5720s and a license would be a reasonable solution, with one unit for the "base" and the other for three field teams. Keep in mind, though, that the minute a radio drops over a ridge, it and its user are out of touch.
Anyway, in my view, it's better to eliminate the need for a radio through good group management. Fine, they're teenagers and I know that poses challenges. But here's my advice: At the trailhead, slip a wireless phone and spare battery into your pocket or pack. Don't let anyone see you do it, but that's your backup comm device in case there is a need for medical assistance. Then gather the group together and lay down the law: Stay in touch with group leaders. Don't wander off. Obey the rules. Then point to an adult member of your group and tell the rabble-rousers: "John here is our designated troublemaker remover. If any members of this party do not obey the rules, he will escort them to the trailhead. There, the parents will be called and asked to pick up their son/daughter." Convey the same information to the parents beforehand.
But, you know what, I wouldn't worry too much. Somehow, Boy Scout groups and outdoor clubs and church hiking groups have all managed for decades without radios and walkie-talkies and sat phones and wireless phones. Even with the "Can you hear me now?" guy crawling all over the place, I'm pretty sure that's still possible.