That flag was from a squad in what turned out to be the last batch of trainees, entering the school in two sections, one April 1, 1945 and the other June 1, 1945.
The school was burned to the ground in an aerial bombardment on June 10, 1945, with 182 students killed.
It belonged to an enlisted men's unit. This unit was a pilot's training unit attached to the Yokosuka Naval District (which is where the 横志飛 on the flag's far right comes from) ある海軍航空兵の手記
Some background info on the incident and and photos of related cenotaphs. 関西甲飛十三期会HP/英霊譜・土浦