Frankly, he could've been a lot more... well, effective, efficient, kind, clever etc. by just acting and seeming to want it as a repetitive thing, rather than it being something that happened, which she'll probably feel pretty bad for, but not understand how it happened, and by making her want it more as well.
But acting that way... just not efficient or long-term, nor really like a standard teenage boy would do (given the amount of hormones, most would jump at the chance to keep it going, even more so due to the teacher being a beauty and the sex probably being pretty pleasant)