Why are Kase-san's classmates all jerks?
She is on a competitive sports team that demands discipline and a high degree of subordination of the individual to the team goal and mission. Insofar as Kase is allowed to deviate as much as she does it is due to high performance. It doesn’t look like a fun environment, but it seems culturally realistic to my experience with such teams. Keep in mind that we haven’t seen any actual hazing that might also show up for such a team if things were really off course.