Lamentations
They were all he ever talked about.
Rachel did this...
Finn did that...
I'm so proud of Puck...
We heard it all the time. He never stopped talking about them, even though they graduated almost five years ago. He didn't seem to realize that we were the New Directions now. They'd graduated and moved on to bigger and better things, and we were here.
We worked hard, but it didn't matter how bright we shine or how well we sing; we'd never be them. We'd never be Schue's kids.
This is a bit of a "What if..?" scenario. Mr. Schue just really seems the sort to get stuck on the past.