You have a point, but according to the timeline I get from the game which I mentioned in previous comment, there must have been some time period between these two events: Susan knowing about LEah not taking the pill and sharing it with class, maybe months or more ? (Leah hadn't met Susan until after breaking up with her BF) Leah would have stopped taking birth control by then since that relationship was over. Also we have to keep in mind that the relation between BF and Susan would have started before Susan knowing this about Leah from Leah's BF (if she came to know it this way ie), her BF wouldnt have just come up to Susan and said teacher my GF doesnt fuck me even though she is on the pill, he must have had an intimate relation with Susan by then to have shared this info.I kinda see both sides of this birth control debate - I agree with yandex that it would be super unlikely Leah's mom and dad would be ok with her being on birth control. OTOH, IF SHE WAS, since Susan stole Leah's boyfriend she could have inside knowledge about Leah being on the pill if her boyfriend blabbed about it. "I'm so frustrated and I don't know why she won't fuck me. She's on the pill so I wouldn't get her pregnant..." "That's ok, come here, stud - we'll take care of your issue."
So by the time Susan shared this with class, even if it was accurate and through her BF (an if), this would have been old and obsolete info.