That's a side of Kaoruko we don't usually see. She looks cool when focused, did that make Rintaro's heart skip a beat?
The difference in academic standards for their respective schools are quite the surprise for these students. Saku's probably that super smart guy whose family couldn't afford to send him to a prestigious private school, so he ended up in a public school. He can keep up with Kikyo-level standards, but is hopeless at teaching people.