I want to also second flying Norwegian if they don't have to worry about miles, etc. I've done the nonstop from LAX to LGW and it was perfectly comfortable and fine. June-July-August is very busy in Europe. If it were just two, they could probably wing-it in terms of hostels, but they might want to book ahead if they all want to stay in the same dorm. If that doesn't matter, they can probably wing-it.
Make sure they have a clear budget that is doable for each of them. They should also plan what sort of things they want to do. Museums? Hiking? Drinking? Etc. Do they have interests in particular things like Roman history or house music? That goes a long way into determining where they should go.
Depending on how long they are gone, they should probably unlock their phones and get a SIM card, unless they have T-Mobile. Relying on WiFi is OK if they have a clear itinerary, but if they're not terribly organized, being able to call/text each other will make life much easier.
Also, while trains are nicer, for high school boys, buses are much cheaper and easier to book last-minute, and planes in Europe are very inexpensive if they want to go to different places.
I'd recommend Lonely Planet's "Europe on a Shoestring" -- it has a lot of good suggestions on attractions and good advice for most countries.
As for safety, much of it is common sense stuff. Know the emergency number(s) for the country you're in. Always keep your wallet with you. Don't flash a lot of cash anywhere. Be careful in pickpockety areas. Since drinking ages in Europe are lower, I'd say they should make not to overindulge. I'd also make sure they are all signed up for the
State Department STEP program, in case of natural disaster, etc. I personally like having a flash drive with emergency info (like my In Case of Emergency contacts, insurance, medication/allergies, etc.) in my wallet, with an encrypted file with information in case I need to cancel a credit card, lose my passport, etc. Also make sure that all of the parents have photocopies of their kid's passport.