She can skip class because of a scraped knee?
I think not. I think it's just Yoriko gay panicked splurting semi-nonsense. Either this or japs schools have protocols that require treating all injuries as "possible fatal until checked". This could explain why many mangas usually make people with even small injuries, that happens on the school premise and in public events, even a literal cut on the finger (or a knee scratch like in this case) , going to see the infirmary's nurse (nothwithstanding that often the nurse isn't even there).
Not sure, many of my country schools don't even have an infirmary or a dedicated nurse. At best some schools have some personnel trained in Basic Life Support, some have AEDs (more schools have one now respect on when I went to school, university had more then one, )