Why didn’t the tomboy want to leave? Is she being abused? Does she like the infirmary girl?
I like the ambiguity in this one because it leaves those very questions open.
Assuming the narrator (Tanaka) is correct that Sasaki is popular and well-liked, she probably isn’t being abused.
Sasaki says she’s never cut class before, so theoretically she just feels like doing it today (maybe inspired by Tanaka’s example).
The visuals, however, suggest that Sasaki does in fact want to spend more time with Tanaka.
Those visuals make her offer to give a reason for staying and Tanaka’s refusal to hear it very interesting—does Tanaka not want to hear the reason because it might not be “Because I want to spend more time with you,” or because she’s one of these “I just want to be beside my loved one but not have an actual relationship because reasons*” yuri girl?
*One potential reason being the problem of an (apparently self-imposed) social outcast falling afoul of the tomboy’s fans.