Michael Torke - Adjustable Wrench
John Adams - Short Ride in a Fast Machine
Steve Reich - Music for a Large Ensemble
Kids should be able to get into the spirit of these pieces quite easily. They should be able to see the references that occur in dance and rock music.
The focus of harmony and rhythm should be the focal point when discussing these pieces. You should manage to get better answers than "I like it because it sounds good".
Also, I would be wary of your term happy, I realise as a teacher you have to keep your pupils interested, but you don't have to do this by throwing Dance Macabre at them all the time.
I also recommend Joan Tower's "Fanfare No.4 for the Uncommon Woman" - this is highly riviting and should be compared with Adam's "Short Ride in a Fast Machine" Play it loud and really get them immersed in the sound of the orchestra; the pupils should be encouraged to engage in debate so they can develop their skills in musical perception, appreciation and purely for the sake of loving the stuff!
Stravinsky's "Symphony in C" makes for a great depature from the boring Mozart pieces. Discussion should be encouraged here also and the pupils should be aware of how the piece ends and this can be a great area for discussion.
The reality of the situation is that most young people don't like much classical music because they view it as snobby and pompus and people listening Mozart and Chopin - show them that classical music can be much more aggressive that heavy metal!!
Bach is fine on some levels, but most youngsters just turn off. Please try the pieces I have mentioned. I'm confident that this combined with some enthusiastic discussions and debate, you'll have the kids eating out of the palm of your hand, they'll be more open minded and then you can throw in some Mozart, Bach, etc!