I used to be one of the staff taking pupils cycling during school activity days. One of us had the first aid kit. I had the toolkit and some spares. We always gave the bikes a thorough check before starting and have had to send some pupils home with unsafe (usually it was brakes) bikes. We usually had at least 4 staff to about 30 pupils.
Only on one occasion did we have an abandonment, Chain snapped and destroyed derailleur. We had someone at school collect the pupil and bike with mini bus.
It would be useful to have some form of recovery either the rider themselves, taxi or someone you can call on. It all depends on your riders and their age, competence, experience, and expectations.
Are you charging for these rides and what are you telling your participants they can expect to get from this? e.g. Sight seeing, exercise, learning traffic riding skills etc.
On one ride we had a pupil who couldn't even start the ride. As we were going through the checks a guy comes up demanding his bike back. The pupil in question, not having a bike of his own, nicked one on his way into school.