In retrospect, you seem to be right, Sensei takes Makoto's virginity before "There is Nothing", during the first Halloween event, so one of the Checks would be hers if it's a sex tally. Take out Sekai, and replace her with Makoto makes the first 3 checks Maya, Ayane, Makoto, and optionally Ami. Later on it could be Maya, Ayane, Makoto, Chika, Kirin, and optionally Ami. That's still only 6 instead of 7 though.. might not be a sex tally at all, or at least not just about sex.
6 seems to be the most recent known default while 3 was the initial default, so 3 notable and unmissable things had to have happened between "There is Nothing" and "Life of Prizes", and getting with Ami seems to just be the extra mark.
Overall: one would probably have to replay the game to figure it out. If it's actually possible to be figured out at this point, and it's not actually including things in the background that we wouldn't know at this point.