Syndication requires 4 seasons. At least for American prime time shows with 20+ episodes per season.
And some community colleges offer 4 year programs. Jeff for instance is getting his law degree there. Most likely, Shirley is getting some sort of business degree and Pierce is just taking classes for fun. Logically, students as old as Brittany, Troy, Abed, and Annie would be in a transfer program so they only have 2-3 years.
They probably will stick around taking electives or whatever to keep the show running. It's not like this is Breaking Bad and a main character has to die soon.