I'm pretty sure I read this on fanfiction.net.
Anyways it is a Kurt-centric fic. I can't remember exactly who he was paired with but I am pretty certain that it was either Puckurt or Klaine. And I think Harry was with Draco. Harry, Dudley, Draco, and any other HP characters were older than the glee teens, if I remember correctly.
Anyways it takes place at McKinley not Hogwarts. Harry and Dudley (along with significant others/other family) come to McKinley after learning about how bad the bullying from Karofsky has gotten. I remember them both being very protective of Kurt and there was a scene where they save him from Karofsky.
Another big thing I remember was that mpreg was included in it. Harry was pregnant and told Kurt that he can get pregnant and I think Kurt is at one point in the story.
Any help would be very much appreciated!! Thank you.