Occam's razor
She was never blind in the first place; it's all a ruse.
If the person who tackled the shooter in the lunchroom looks exactly like 'hap', then there is no 'hap'. This was the high school she attended before she disappeared; she would have had to have been sighted to see him.
She is a psychopath. If she could convince those five people to believe her story and learn that ridiculous dance, perhaps she has also been talking to the shooter over the past few weeks, and convinced him to shoot up the school.
Also, her face is not visible in the YouTube video of the violin player. Is that really her?