They're really acting like it's a heavy sin when it really isn't. Are teenagers that dramatic ?
you can’t really say that it wasn’t heavy for Ruri who went through Shizuku’s harsh comments thrown constantly at everyday during their elementary school days. To Ruri it was a lot and we don’t know her personal history of why she wasn’t at school for more than half the time. It takes a lot of growth that Shizuku understands the turmoil that she put Ruri through back then.
This manga perfectly encapsulates that we humans are flawed, and we are learning to apologize/take accountability for our actions. Sometimes we don’t get the grace/forgiveness after apologizing to someone and that’s okay. At the end of the day you have to learn to forgive yourself and your past actions which Kaori is teaching Shizuku about