I had to tree up and she had to queue for green eyelids.
It's an unusual phrase but it makes sense because you have given us the context of the school play.
It means to put on the costume that makes him look like a tree.
The usage is similar to a phrase like:
"He wanted her to glam-up for the big night out" – to make herself look glamorous
I would be inclined to use a hyphen for both terms, tree-up and glam-up.
not a teacher