For the very first release, I knew I wanted a variety of models for the teachers/students, so I mostly used pornhub (and later pornpics) to sort all models by popularity and ethnicity/hair color, and then I'd just pick people that I thought looked the part. People like to make fun of Sara Jay, but as the trashy alcoholic mother of your childhood best friend, I still think she's perfect.
Other people were added as the game went on because of requests that people made (like both Rachel's regular and bimbo forms).
Nowadays, for my games/stories I usually just have patrons vote on which model(s) they'd like to see and then work them into the narrative as appropriate. But yes, even then I still try to check how many clips are available so that I don't have to rely too much on stand-ins or recast them (like Lydia's younger sister and I believe Ms. Ward's too).