Chrystal:

Tier 1:

 Have her come to your house for tutoring and when all the scenes have been played talk to her at school to have her pick up her new outfit on the way to your house.
 There will be a scene at you home and after that you should find her at school again.
Tier 2:

 You talk to Chrystal at the school to find out the Dean has put a stop to the tutoring sessions. So go find the dean at her office and talk to her. Make the blue serum and use it on her, then talk to her the next day to convince her to let the tutoring continue. you can then talk to Chrystal and resume the tutoring.
 Get her affectionate enough to be able to use the green serum on her then meet her at school.
Tier 3:

 First meet with her at the school and talk with her, then meet her the next day at school again to be able to invite her home.
Dean Kara:

Tier 1:

 Seeing at her first encounter ties into Chrystal's story first order of business is changing her mind with the blue serum.
 Meet up with her again to set a date when the school is out (From home in the evening you can choose to go meet up with her at the office.)
 Do enough of her actions, then use the pink serum for a new scene which is triggered by you going to meet her the morning after at her office
Tier 2:

 Hangout with her at her office for the encounter with Howard.
 Check her office to be prompted to make a call to her to have her meet you at your house.
 Talk to her about the situation and she agrees to a makeover.
 Call her again later to have her come to your house.
Tier 3:

 Finishing all the available actions with her at that stage should open up two new actions in the talking options. One is dealing with Howard, the other is dealing with her ex-husband.
Yuki:

Tier 1:

 Train with her until the option to use the serum becomes available.
 When at home at night the option to check on her via the phone becomes available, you exchange some messages and you can then talk to her at the school.
 Talk to her at school and convince her to help you out at your office (Go to lab, then to office from there)
Tier 2:

 Do different tasks with her until you have the option to make her change her wardrobe.
 Her new clothes can be bought at the adult shop.
 After you give her the clothes you have to meet her again at the school in order to get her to meet up with you again in the office.
 Do some more actions with her until you have the option to test the green serum on her again.
Tier 3:

 Having done that, meet her at the school again to see her new look, follow the story and then have her come to your house this time for some playtime