Sou mistakes it for a confession?
And were, exactly, would be the mistake then?
The bigger question is how she will approach him. She consented into getting regular hugs from Sou, but the guy himself has no clue about it. And bet your ass, no guard team would allow a girl to come near Sou and ask for something like that. It's one thing if Sou initiates it, but from the girl's side? That's pretty much sekuhara in that world and could get her expelled. What Hana did in suggesting the hug fest was borderline - in fact, it crossed that line, and she can be happy Sou was chill. But back to Yuri and her strategy to get her dose of Sounium (Dar-phemo).
I have no clue what Sou heard from Yuri inside that locker. He thanked her, so she must have said something useful. Maybe they made arrangements to meet later, without too many prying eyes/ears. Whatever happened could change the way Yuri can go at this task. Let's assume she does not have his social media contact and doesn't know where he lives. Then she cannot do much for now, because Sou has already left.
Under that condition, if I were Yuri, then I would go home and put my housewife/girlfriend skills to use. Meaning, I'd prepare a batch of tasty chocolate sweets (cookies, pralines, whatever) and wrap them cutely in a small bag. Yuri has plenty experience in cooking and baking, she should be able to pull something off that would be delicious. Then she could hand it to him directly next morning or slip it into his desk before he comes to school. One way to be alone with him for a while would be putting a note in between that says "Let's meet at our place after school (Yuri)" (= old school restroom)...
If she knows where he lives, she could visit him directly - questlon is how she will handle Homeland Security (the three succubi). Not sure if Sou in this timeline would be as bold as inviting Yuri to his room.