In my opinion, she has OCD. The condition is not severe,though.
Are we taking a poll here? If so, consider the scene early in the film when Shirley is looking for a lost book. You see a ridiculously clean and ordered room, with neatly spaced and lined-up post-its outside her closet and neatly stacked boxes with clothing inside as well as inside shelving, lined-up artwork on her wall, books lined up on shelves with bindings flush with one another, a blackboard with days of the week marked and space for items, etc. You even see her move a stapler to line up parallel to the desk side. Especially for a teenager, usually considered a model of messiness, this is remarkable.
