How do I unlock the scene seen in this picture?
Use your power of Control Magic to become a Hero... Or Villain! · By
You need to have enslaved Sarah for Cass, either by dating Sarah as Dan and then getting a scene where Sarah confronts you in the streets about Cass's control over you and choosing to side with your mistress, or enslaving her as Rosa, putting her in Mindless personality, and doing the Library play with scene with Sarah, then have 3 or more slaves of your own, for Val to not be one of those 3, have had the prompt to give up one of your slaves, have given up one of those slaves, and then getting Cass to ~level 20 and encountering a scene in your bedroom.
Teachers don't count as slaves you can give up to Cass, but do count towards the 3 you need for the first scene after giving up Sarah or helping Cass enslave her (depending on if doing the Rosa route or the Dan route). Only other students, and the non-Adult NPCs, count as ones you can give up to Cass.
Also if you get Cass to Level 20 before having gotten 3 slaves of your own after having given up Sarah, you'll get a trio of extra scenes where Cass laments how boring it is only having you and Sarah as slaves, and if playing as Dan, leads to a way to swap to being Rosa.