Also the other mysteries at school are probably Hanako-san just playing around.
Could also be that the effect of her powers depends on your mindest.
Like if you approach the staff room during the day when you're relatively relaxed and think it's reasonable for teachers to be around it's super easy for her to suppress your feelings of "wait, what class does Hanako sensei teach" and convince you she should be there.
But if you're approach the staff room at night in "walking through dark empty place at night, isn't this a good time to remember horror stuff," mode and you aren't expecting anyone to be around the suppression fails so you're like, "where the FUCK are those footsteps coming from."