in my high school years, i was lucky enough to visit 11 countries on 4 continents. briefly, and in no particular order-
Japan: do not make fun of punks on a train, just in case they speak english, get off on your stop and ambush you in an alleyway. don't eat the fugu, it might kill you.
China: state sponsored tour guides will lie to you at any opportunity. bring an independent for facts. also, things might explode or collapse at random, from chairs to bridges.
UK: the food is bland, they won't admit it but you shouldn't comment on it. avoid speaking any foreign languages you might know.
France: for the love of god, don't try and speak French to them. you'll be offending their delicate sensibilities, and then you're just screwed.
Greece: the cops will be upset with you if you report a crime. also, WATCH YOUR WALLET AND STORE YOUR PASSPORT IN A SAFE. your life might depend on it.
the Netherlands: crazy place. don't bring children. don't confuse coffee houses with cafes, as they are two separate things. take a canal ride.
Turkey: was glorious, but things have changed and now I would worry about ever going back.
Australia: everything, from the wildlife, to the ground you walk on, to the water at the beach, to the trees you see, probably has the potential to kill you. read up on EVERYTHING.
New Zealand: if you plan to stay on Maori land overnight, maintain eye contact with the guy who greets you and let him turn his back on you first. you'll insult him if you do either before he welcomes you, and then you have to make your way off their land on your own.
Mexico: if the army shows up and says move, MOVE. if you're a woman and arrested, don't be surprised if you end up in a cell full of Transvestites. its so the women in the womens cells won't beat you to hell.
Canada: is lovely. enjoy.