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First you have to reach level 4 (you and Val). Next you have to talk with Ms. Rack, having Val in her Bimbo form (she will ask about Val odd behavior). After that you have to reach lv 14 (you and val) and, with Val into her Bimbo form, talk with Ms. Rack again. When Ms. Rack asks you about Val's behavior, lie to her, then don't ask Julia for intel on her. In Ms. Rack's room, a scene will be available where she asks you to try to convince Val to stop acting like a bimbo. During this scene, you can use the Flirt spell to trigger a sequence where Ms. Rack challenges you to prove you can "handle her".

You have a choice of where to focus your efforts pleasuring her; but both will end in failure on their own, and cause Ms. Rack to reject you. In order to succeed you will need to cast the boost spell on her before the scene ends. After talking with her, go back into the classroom and Val will turn her into a Bimbo: offer to fix her.

After that, go back into your house, turn Val into a Bimbo and talk with Ms. Rack again: now you will unlock the Bimbo world ending