YA character becomes invisible and spies on classmates

[Wendy]

I think she ends up spying for her classmates, and that being invisible is a diagnosis/illness.

[Becca]

It might be on this Turning Invisible/ Invisibility list - https://www.goodreads.com/list/show/1...

[Kris]

Wendy, what's the genre or tone - e.g., YA fantasy, mental health, drama, humor, etc.?

Does the girl actually become invisible or does she just imagine she is?

Around what year did you read this book?