How long ago were the flashbacks?
These are the 2 scenes you probably mentioning in your question.



1) You can re-watch episode 8 of K-On!!, roughly at 07:20, Nodoka answers Mio that she first met Yui in kindergarten. (Approx. 12 years ago)


2) In the same episode, at 12:38, Ritsu relate her first impression about Mio back in elementary school. (Approx. 8 years ago)


3) The actual year is not confirmed yet, maybe plus minus a year or so.