Class of 2012 (sophomores in Season 1, graduated at the end of season 3): Rachel Berry, Finn Hudson, Kurt Hummel, Mike Chang, Quinn Fabray, Santana Lopez, Noah Puckerman, Mercedes Jones. Note: Brittany S. Pierce was supposed to be a part of this class but it is revealed in the season 3 finale that Brittany has to repeat her senior year because she did not pass any of her exams.
Class of 2013 (freshmen in Season 1, graduating at the conclusion of the school year that began at the start of season 4): Tina Cohen-Chang, Artie Abrams, Blaine Anderson, Sam Evans, Brittany S. Pierce. Note: Due to actress Heather Morris' pregnancy, her character (Brittany S. Pierce) was written off the show (at least as a series regular) as graduating halfway through the school year.
As of season 5:
Seniors: Tina Cohen-Chang, Artie Abrams, Blaine Anderson, Sam Evans
Juniors: Joe Hart (assumed), Sugar Motta (assumed), Wade "Unique" Adams (assumed)
Sophomores: Marley Rose (stated in Season 4 season premiere), Jake Puckerman (stated in Season 4 episode Wonder-Ful) Kitty Wilde (stated in Season 5 Episode 2 Tina in the Sky with Diamonds), Ryder Lynn (assumed)
Season 4 ended with the regional competition. The beginning of season 5 will presumably pick up with the conclusion of that school year, notably the national competition and the graduation of long-time Glee club members Tina, Artie, Blaine and Sam.
As a side note, the fact that some shots have included Glee club members in the same class (like Mercedes and Santana while singing "The Boy is Mine") is not concrete proof that they are in the same graduating class. Under the American school system, it is possible to be of different graduating years and yet be in the same class together.