Following up on this, there actually might be a clue that Ami knows about the journal that Sensei hid. When Sensei is catatonic in "Not the Nightingale", she mentions the attic and Sensei's "secret hiding place".
This doesn't necessarily mean the hiding place is in the attic, but it's suggestive especially since I can't remember any reference to Sensei hiding anything else in the house (hit me with alternate theories, though).
This raises a couple of questions. The first is why this detail would be important, and in particular why Sel would have Ami only hint at it instead of revealing that she read Sensei's poems as well as Sekai's. Maybe Sensei hid stuff other than the journal in the same spot, like mementos from the accident. Or maybe there's incriminating information in the journal, either about Sekai being a monster or about Maya.
The second question is why would Ami bring up a secret compartment under a floorboard in the context of protecting the house from intruders. It doesn't make much sense, but I'm not sure why any sort of hiding place would be relevant.