I engage the bag packers in conversation, advise them how I want the bag packed and give 'em some dosh. After all they're kids learning if they want something they have to do something for it. Some causes are perhaps more worthy than others but the principal is the same.
it's also not always obvious what's going on. My son did a bag pack for a S.Africa trip. On the face of it it looked like they were collecting money to go to S.Africa and have a nice time but in fact they were collecting money for the charity that the school supported and through donations, effectively paid for two permanent full time staff and many projects. He and everyone else paid his own way to get there.