If the characters were to have a birth year, it'd be pushed back and change every year.
Going on the 2011 calendar on the wall in the first anime episode and Miki stating that she was in ninth grade at the time, I very much assume the following birth years for the characters:
Azusa — Thursday, 19 July 1990
Ritsuko — Tuesday, 23 June 1992
Takane — Friday, 21 January 1994
Haruka — Sunday, 3 April 1994
Makoto — Monday, 29 August 1994
Yukiho — Saturday, 24 December 1994
Hibiki — Tuesday, 10 October 1995
Chihaya — Sunday, 25 February 1996
Iori — Sunday, 5 May 1996
Miki — Saturday, 23 November 1996
Yayoi — Wednesday, 25 March 1998
Mami & Ami — Friday, 22 May 1998
I guess that makes them all millennials.
Anyhow, I share a birthday with Ikebukuro Akiha, my own birthday being 10 June.