As the owner of the premises, the school has the duty to exercise ordinary care to keep the property in reasonably safe condition. Whether or not they would be found liable (or partially liable) in a court of law depends on the foreseeability of the injury. Was it foreseeable that by not locking or placing a warning sign on the attic door that a student would enter and fall through the ceiling?
Other considerations include whether or not your daughter was negligent herself in exploring an area which may very well have appeared to be 'off limits' or difficult to access. A finding that your daughter was negligent could reduce or eliminate any damage award.
You'd be well served to consult with an experienced personal injury attorney in your area. Injury attorneys often will agree to meet with a prospective client to review their case for little to no cost. Best of luck to you.
Disclaimer: This does not constitute legal advice. No attorney-client relationship has been formed through this answer.