First is Sensei's age, as Yasu apparently is told a seemingly impossible number. Maybe it's an insanely large number if her God accumulatively counts all the time "Sensei" spent during countless resets, or a single digit number since "this iteration" only went through this much of time.
My reading was that it was an impossibly large number because of the time loops.
Or an even more impossibly large number because he's a god (theory only at this point)
The other is where Sensei used to tutor. Yasu knows several places, and two of which are very naughty. It's more likely it's the tutor and his students being naughty rather than the location itself, so this might be pointing at Maya. What I am wondering is if it's simply Sensei and Maya did naughty things at two different locations, or if there existed another person Sensei was doing naughty things with.
IIRC, Noriko actually mentions that the dingy apartment he tutored her in was sketchy, so it's very possible that Yasu is speaking literally -- he may have rented room in a building that served as a brothel, sex shop or something similar.
And we're so close to find out who Sensei really loves! But it's probably just Sekai if Yasu decides not to say 
Yasu's comment confirms for me that "Sekai" is not her real name. If bad things would happen if Yasu said the name, then bad things would also happen if Sensei READS the name, which means the name on the gravestone is not her actual name, and "Sekai" is most likely a term of endearment.
Interesting note -- I replayed the scene before answering, and it cuts off completely prior to the question about Sensei's mother.
Which means that question only gets asked when certain flags are set.
That isn't something I expected to be flag-dependent.
