There are not many places to discuss this series and I wanted to discuss this chapter.
My theory is that this is a fake memory that she gained at the sanitorum, and there were orginally 9 kids and 1 died in the courage test. There are many things reinforced through the chapter.
There is an enfasis in there being 8 of them (the weird call from the profesor, 4 bunk beds so 8 beds, 2 panels showing all 8 of them, etc).
The courage test was called off for a weird reason, this seems like the lie to forget that there ever was a courage test.
The kids chant "there are 9 of us" while in a sleep talk. This might be what they told each other while in the sanitorum to not forget the dead kid.
And finally, MC wonders specifically in what she forgot.