I was a P2P ambassador back when I was in high school. This was back in the late 90's. I did the Western Europe tour for 3 weeks. Our trip was all high schoolers ranging from 14-17. I was almost 16 when I took mine. We were closely chaperoned for the majority of the trip, however, there was a 3-4 night period where we did a "home stay" with a host family in Germany. They split us up into pairs and we were assigned a random family. My first "family" was an older single woman who barely spoke English. I contacted the chaperones/tour guide and was quickly reassigned to a family with a daughter my age that spoke fluent English. It was fantastic. However during that home stay, being a teenager in a country with different laws than the US...well...yeah, we broke some of the trip rules...
Anyway, the 3 chaperones that came on our trip were actually teachers in another local school district, we had actual classes that were mandatory to attend ahead of time and those covered everything from the cultures we'd encounter, to a "how to" for traveling abroad, etc. We also did fundraising so the cost of the trip was cut in half when said and done. The actual "tour" itself was provided through a tour company called Contiki Tours. I don't know if that is still the case?
If I had been given the opportunity to do it again, I would have in a heartbeat. It was an amazing eye opening experience and I am forever grateful my mom allowed me to go on the trip. Granted, I was older than middle school, so that's definitely a consideration. I couldn't imagine letting either of my children do a trip at such a young age, but high school is a different story.