There's the LN for this. In fact, this manga is the adaptation from the LN. There are 4 volumes of it and it's already translated by the same group who scanlated this manga. It's worth reading if you only focus on Iroha, Oz, Sumire-sensei, and Aki. Mashiro can go to hell for all I care.
What happened to Mashiro and why she is like this is because of the bullying from the her previous school. But even so, her treatment to the MC just way too much even for a tsundere. That just being mean and cruel. Really, in reality, under normal circumstance, doing that is a recipe for living all alone for the rest of your life.