I think you're making a very bad decision in going with "Drapery".
Firstly, the song relies pretty heavily on ambience, which will be difficult to replicate with a (forgive me for being blunt) green band playing on a school stage.
Second, it's a slow song by most peoples' standards. I'll be surprised if you don't loose 90% of your audience's attention during the first vocal section. Once you loose them, it's hard to get them back.
Thirdly, "Drapery" is something of a difficult song to grasp. It was the last song on the Blackwater Park album that I came around to liking; for a long time my attention would drift elsewhere whenever the track came up, even though I loved the rest of the album.
Last, "Drapery" is a grower. It doesn't actively seek out the ears and scream "LISTEN TO ME!!" It sits patiently, knowing that the listener will come around sooner or later - but that's assuming the listener is going to to hear the song quite a few more times. "The Leper Affinity" is a song that grabs you by the throat and makes you take notice.
All things considered, I have a feeling your school will have things on their minds other than your music, at least during the "Drapery" section of your show.