couldn't she grab clothes and dress outside?
Her clothes were not between her location and the planned fire exit. In fact, they were probably in the girl's locker room which would have meant walking into another building, fiddling with a lock, etc. I think going immediately to the fire exit was absolutely the right thing to do.
The screw-up (and a MAJOR one, at that) came once she was out in the freezing weather. Some responsible adult should have seen to it that anyone coming out of the pool was moved to a safe location. The failure to plan for students clad only in wet bathing suits and no shoes in freezing weather was a monumental goof on the part of the school administration. The failure to take immediate action to keep those students uninjured was another monumental goof on the part of the teachers and other staff present. This should go on several school employee's Permanent Records!