This line represents an important shift from what the speaker wants them to do, to what "they want to do."
Collins uses a technique called enjambment to build a little suspense here. We don't find out what they want to do until the next line.
is tie the poem to a chair with rope
Whoa! Things have taken an ugly turn. We've seen this movie, and it doesn't have a happy ending.
It is important to note that the poem is being restrained, restricted. The poem is tied to a chair. Fun's over.
Whoop, Whoop! (Okay, no more sound effects. That was the last time. Promise.) Collins gives us another metaphor here. The poem tied to the chair is a metaphor for all those things you think it would be a metaphor for: restriction, loss of freedom, lack of movement, and just about all things negative and ouchy.
Bad things are coming in the poem and Billy wants to make sure that we get it. We do.
and torture a confession out of it.
See? We knew things were heading in a nasty direction. Torture! Bad!
They (the students) are trying to torture a confession out of the poem—trying to get the poem to give up the goods. They want the poem to confess and they are willing to use any means necessary.
While those of you who have taken Mr./Ms. X's (insert name of toughest teacher at your school) class might argue this point, torture has no place in academic pursuits.
Seeking truth is good. Blindly seeking answers is bad.