I would assume the sounds would be copyrighted. And since this is a profit event, even for a school, they probably wouldn't give you permission. However, whats the chances of a rep or attorney from Lucasfilm actually going to your variety show?
You could make your own, and since I certainly hope you don't have a 5.1 system in your theatre it wouldn't be too hard.
Ya, my guess would be that they are copyrighted too, but I would just make your own sounds that resmemble those sounds. Like Aaron said, what are the chances that an attorney will be there, and if you make your own sounds, even if they sound quite similar, you can probably get away with it.
(I know you can freely download movie clips of several of the THX intro videos, you might want to happen to download and watch that video on your computer while it is plugged in to your school's sound systemsystem. I realy dont see huge legal problems with that b/c it's a free video, and almost everyone will understand that it's just a joke type thing)
Just a bit off topic -- but all through our production of macbeth, my sound check routine every night was to play different veriations of the THX sound through all six speakers around the auditorium at once. I played the original THX sound the first night, the robot one the second, the simpsons one the third, and the 'modern' one on closing night. It was good times.