Sarah needs to have been dollified (meaning you've turned her into a doll through the Stone Skin spell (evidenced by the presence of doll joints on her arms in her portrait)), and must also *not* be a slave. At that point, after getting to the "you can have Val bimbo one person at a time" state of Bimbos Gone Wild, you'll see Sarah's icon in the school hallway, and then after talking to her there, you can find her in your bedroom so long as she remains not your slave, where you can bimbo her (the only case where Doll Sarah's portrait will change at all from personality alterations, as no other personality has an alternate version for Doll Sarah).