Old comments, but for everyone asking why the mom let the teacher up to her room, iirc its not too uncommon for teachers to make home visits in Japan, its part of the job sometimes. Like, in the US we had those "parent teacher conferences" where parents could come up to the school on a certain day to discuss their child's school work, in Japan teachers come to the students houses instead. For a lot of schools (middle especially) there's certain days out of the week after about 2 months or so of the start of the year where students have half days so that the teachers can go make their rounds, meet with parents at their homes, and get a better understanding of the child's home life.
Granted, a home visit like the one he's doing here is usually more for students who have larger gaps in attendance than just a sick day, but since teachers making home visits is a normal thing in Japan I can see why the mom didn't think too hard about it. Yeah, letting him up to her room might have been a bit strange, but keep in mind she didn't actually leave them alone long. If he hadn't already assaulted her (which her mom has no idea about) he wouldn't have even had time to do anything to her with her mom so close by. All he could do was verbally threaten her with a preexisting picture (that again, her mom doesn't know exists.) Her mom also believes she's sick, so she probably didn't want to drag her out of bed to come downstairs to talk to her teacher, who probably said something like "I just want to explain a tricky subject we learned today to make sure she understands the instructions" or something.