@Agni I mean unless she was faking her name he looked it up in the missing children database. It seems there was a lot going on and if a parent does not even care to report a child missing I doubt they are looking for them. She said 6 months which means it's not like it's been a few days and it seems so far it's been a few weeks that she has been there.
I will go into story time, my mom had(has? Complicated.) a friend who is around her age so late 50's now and they used to work together decades ago. She has a daughter who is about my age and we played some when I was young. Well the mom was a fairly consistent drunk and a lot of relationships, maybe some prostitution? Never got much of the answers of what she was up to when I was young, my mom seems to be a magnet for these kind of people, but that is a whole different story. Anyways, the father didn't want much to do with the mother for good reason but did try to take care of the daughter, the parental rights were split between the grandmother from the mom's side and the father. Because kids often look up to their parents regardless of if they should the daughter ended up with a rather warped sense of self leading to her rebelling against the grandma/father who were trying to be strict so she did not become her mother and a lot of running away and while she did have friends, you can only wear out that welcome so much before you aren't welcome anymore. That was all after I had moved away when I was 10 which lead to the mother moving down where we lived, like 1000 miles away and she started freeloading here which is around the time the daughter started rebelling more. The daughter eventually ended up down here after the mother had moved out from staying with us. The daughter still rebelled and ended up on the streets finally since new area and few friends which lead to her doing stupid shit and in juvie.
At this point you would think this is where the story ends but no she gets out and the mother pretty much was cool with her daughter doing whatever, so the daughter finds a guy, does drugs, gets pregnant, ends up in prison, gets out, gets pregnant again which is the last I have heard of what happened to her. The mother at about the time the daughter got out was doing harder drugs and selling herself not long after which is about the time my mom tries to limit contact(can't figure out why my mom still talks to her, especially after some of the shit she pulled on my mom).
This story has a familiar path that Sayu could have gone compared to the daughter of my mom's "friend". The reasons Sayu is not pregnant is likely because most of the men did not want to have to explain a pregnant high school girl. She could have VERY easily gotten hooked on drugs if she ran into the correct person, she could have been sold to someone or trafficked to another country(runaways are very susceptible to that). Why is Sayu the way she is? Well it's a mentality of scarcity and fear when you become vulnerable and people start taking advantage of you. She did what she thought she had to to not go back and it's why she can't just accept his generosity. Also, I don't think she could give a shit what the people at her school think or if she was expelled.