For Linda: In the afternoon tell her to follow you to and go to your room, then click on her icon and ask for help, should be one of the first options.
For Brittany: I'm not 100% sure on when since it's a side scene and is random but you gotta go to her house I believe in the afternoon and go to her bathroom where she would be showering.
For Bernice: I'm not really sure how you missed it since it is a main quest scene and would have been part of her quest, can't say I remember how to trigger that one.
For Olivia: Go to her house on a weekend in the morning and go to her bedroom
For Celeste: It's one of the after school activities like the ass job.
I'm honestly not sure how you missed a lot of these since they're mostly story quest and you're told how to get em as you progress.