kid that everyone forgets even own parent. kid is in elementary or middle school. other kids try to do experiments to see what is going on. They make the kid wear a funny hat or outfit once too. everyone forgets him even teacher. they dont know why

[Heidi]

a regular kid at school notices a classmate that he does not recognize and that no one recognizes, not even the teacher. but the kid insists he has always been a part of their class. but everyone has a hard time remembering him. The main kid and his friends decide to experiment with this to try to understand what is going on - but they keep forgetting him so this is difficult. At one point they make him wear a weird hat or helmet. The kids own father forgets to pick him up from school even one time.

i read this when i was younger than 12 and probably before 2010.

[Paige]

The Schwa Was Here
https://www.kirkusreviews.com/book-re...

[Lobstergirl, au gratin]

Heidi, was that your book?