TL the show has spent half of season 4 and season 5 to date in NYC, so this has been in the works for a while. This year's "graduates" from McKinley are the remainder of the original Gleeks, so it makes sense from that perspective.
You may recall that prior to season 4 there was intense speculation about a spin off to follow those Gleeks who went to NYC after they graduated, and those plans were shelved sort of to speak because of the outcry from fans. Instead they divided the show in half for last season and this season (as the school year they're finishing up now covered seasons 4 and 5).
No one except the executive producers Brennan, Ian, and Ryan, and the head of FOX knows what they're planning for the last season of the series, which is why the status of the second generation Gleeks, Sue and Mr. Shue are seemingly up in the air. There are rumors that Mr. Shue ends up somehow in NYC and possibly Sue as well, but at the moment those are just rumors. And the actors who make up the rest of the Glee Club are still under contract through season six, so no one knows what is planned.
Ryan Murphy has described how he had originally planned to end the series with Rachel and Finn who would have been the focus of season 6, and he has come up with a way to end it now with Cory/Finn's passing earlier this season, but he isn't showing his cards.