@Pandawanka Yeah, Rizu's development regarding her most serious issues (largely related to her previously low ability to understand emotions and lack of expressions) was mostly finished in the regular route (though all 5 routes are
parallel ones, each one just as valid as the others). Sawako being featured here has to do with expanding upon Rizu's progress in developing a bigger sense of empathy, understanding how someone else feels and trying to help them. Just because it's the "Rizu Route" ending doesn't mean it has to be entirely focused on her alone, right? While the following routes haven't been released yet, we can't be sure, but I assume not all of their chapters will be solely about their respective girls, to add variety and acknowledge events outside of the essential ones (which would explain how each ending came to differ from the others).
@Yuukecchi She didn't feel comfortable being in (middle) school because all of her (middle school) classmates shunned her for being "too smart". While she probably did attend lessons and/or study properly, I assume most other times she would choose to stay at the nurse's office, a safe place she could easily go to (the only one, really, emotionally speaking) by telling the teacher she wasn't feeling well.