I believe you're talking about the pregnancy ending? If so, I've checked, and you need to get at least 5 creampies in a run. So basically whenever positions like missonary, on-desk and doggy show up, just choose those whenever possible. The bent-over action with the other teacher (Prof. Jenkins is his name, if I remember correctly) also counts as a creampie.
P.S You also need to have seen the true ending eariler before you can actually get this ending.