My friend's son is going on an extended, school-sponsorded trip, 3 weeks, out west. He's not backpacking but camping and day hiking. The question he asked me was, "how many exchanges of clothing should he take, particularly things like underware?" I believe everything will be packed onto the school bus and he's packing a big duffle bag and a day pack.
If it's an all-boy trip, maybe the leader can control it so they need less underwear. I used to call my troop together about halfway through the week at summer camp and tell them, "I've got good news and bad news. The good news is we're going to change underwear today. The bad news is that Chris will change with Todd, Bill will change with Jerry, George..." (Sorry, it's a tough day at work.)