This episode hones in on Satoko's continuing isolation from Rika, her attempts to course-correct, and ultimately opening up the door that will lead to Rika's torturous loop. Now, my knowledge of St. Lucia Academy is limited to Shion going there; I've seen in the comments that it's also mentioned in
Umineko, where its darker side is more evident, but I didn't expect the hoity-toity girls' school to have an
underground prison. That's where Satoko ends up after her attempt to lay a trap with washbasins ends up actually injuring someone.
The scene is reflective of the major differences between Rika and Satoko. Rika has had to live as a child for a century, so it's understandable that she's enthusiastic at playing a fancy lady at St. Lucia that gives her a chance to feel mature for once. Satoko doesn't have that knowledge – she's a traumatized girl who had to act grown-up to withstand social isolation in the village, abuse at home, and the loss of her entire family. Her childhood was taken from her, and that's how you get someone who both puts on airs of aloofness and capability while also being immature and childish. There was no way her cartoonish prank was going to end well, but it also felt like a wake-up call that there are consequences and that this really isn't Hinamizawa anymore.