I have been looking for this fic all week and it is driving me crazy. This fic takes place after Kurt transfers to Dalton, and he is performing a duet to Katy Perry's Firework with Blaine. I think in this fic ND didn't think that Kurt leaving was a big loss, but then they realized they made an error when they heard him sing. I am sure I found this story on this site at one point, but have been through all the tags and can't find it. If someone could help, that would be great. Thanks in advance!
*Found: Link in Comments*