Does anybody remember this book from the 70s or early 80s? Tyke is a schoolkid who keeps getting into trouble. Danny has learning difficulties and is Tyke's friend. There is no indication whether Tyke is a girl or a boy. To my recollection boy readers always assumed that Tyke was a a boy, whereas girl readers were about equally divided into boy/girl/not sure, depending on how tomboyish they were. You find out towards the end of the book.
It's due a resurgence, I think.