Sid's treehouse is still intact even after Lorca arrived. It's true that you're wondering how can Lorca get in and out of the treehouse as well as the School in the Sky while in a wheelchair? We've seen Lorca inside and outside of the School in the Sky in a few episodes.
I and a few fans would guess that one of his friends like Ord would carry Lorca up to and later down the stairs of the School in the Sky and treehouse respectfully, while Cassie and Zak & Wheezie possibly would carry Lorca's wheelchair up to and later down the stairs of the School in the Sky and treehouse respectfully.
