Good ideas so far. In the past I've heard of people working with/partnering with the boy scouts to get insurance. Not sure they are still doing this.
The above comment is spot on about not being their "teacher". Unless you have a formal club, and are insured. Risk management would be all over you.
Finally, it's really important to be ready to pull the plug at any moment with the whole group, or have two vehicles, and at least two leaders (with one of them designated to take any sick or lame home early, if needed). And now days, you probably need 3 in the car if they are minors.....