Get a girl's level ("_____ has leveled up! I bet she has some new tricks to learn") and stats (love, obedience, inhibition) high enough and go to the Professor's office. With Rogue and Emma you just need to bring them in and tell them to do Plan [Greek Letter], but with Kitty you need to break into his office at night and find a photo for blackmail. Getting the photo is heavily dependent on luck and RNG. Once you have the photo and Kitty's level and stats are high enough, you can do her plan the same way as Rogue's and Emma's.
If their level and stats are high enough you can also just do it when you get caught and Xavier calls you in.
If you can tell a girl to do her plan and she just acts confused, you've met some of the requirements but not all of them.
Also, Laura does not have a plan yet, so Xavier can't be convinced to ignore your activities with her.