Well kei's words tell me that she had an encounter with Shinomya before. Whether those word were by cold Shinomya or not means She didn't take the encounter like something serious.
Another thing and I don't know if this has any possibility, but could the mother had remarried to Shinomya's family? (I don't think this could be a thing, but it got me wondering)
Shinomya might go to another University, and I think it is mostly because her family would try to separate her from a commoner. They are a prestigious family and to my knowledge they could influence the college She wants to go with Shirogane, so they don't accept her.
It is a possibility that an engagement is coming her way and It could bring her a lot of issues (I see this as very unlikely tho)
Lastly I think there's something regarding Shirogane's mom that could potentially break them apart that I don't know.