Surprisingly, most school trips don't include onsen anymore (because of the lack of gender segregation, public liability etc) so I don't know if anyone can tell you about it.
"chin chin" is the children's word (as in 5 year olds) - say "o-chin"
I don't know about youth hostels because I stayed with a host family, but your itinerary really depends on your budget and where you want to go (as well as when). If you want a mix of sights and shopping base your trip around a major city, like Tokyo or Kyoto. From here you can see many of the famous temples, castles, shrines etc. Shopping in Tokyo is mostly in the Ginza (for drinking and STUFF), Akihabara (for consumer electronics), Harajuku (for freaky Japanese fashion) and Shinjuku (for...other stuff). The Meiji Shrine and Yoyogi Park in Tokyo are excellent places for a quiet day out, as is Kamakura (outskirts of Tokyo) if you want to see a really big Buddha. The biggies like Mt Fuji, Nijo Castle, and Kiyomizu lie within bullet train access of Tokyo (as do Kyoto and Hiroshima) but to get there would cost a bundle. If you're staying for a week or more and want to see more than a single city INVEST IN A JR RAIL PASS. It costs a bundle (prices vary from year to year, ask your travel agent when you book) but you get unlimited rides on the shinkansen - well worth the cash.
... if I think of anything else I'll tell you.
