Oh, it's a brilliant book. My DDs loved it. It was written in the 1970s, I think, and it was very much how I remembered primary school at that time. There's a wonderful description of Tyke lovingly making her dodecahedron in Maths classes, in the halcyon pre-national-curriculum days. The point of the book, I think, is that Tyke is never given any pronoun, and children assume that she's a boy. Her friend, Danny, is a boy with learning difficulties, and the two of them get into mischief but nothing really bad, all very lovable.
DD1's teacher read it to the class in P6 in the late 90s, thankfully just before social media became a thing, and she didn't tell the children that Tyke was a girl, just let them think she was a boy. The big reveal at the end, when the enemy teacher calls out, "Theodora Tiler, you naughty disobedient girl, come down at once" absolutely outraged DD1 and her friends. They thought the teacher was being massively disrespectful to Tyke by calling her a girl. She then talked about it for weeks, about why to call a girl a boy was such a bad thing, and about why she'd assumed Tyke had to be a boy. If I sound like I know the book very well, it's because DD2 became obsessed with it, and I read it loads of times to and with her. Apart from anything else, it's a beautiful book, and captures the pre-teen years so well, even though it's dated.
Thank you, OP, for reminding me of it! It brings back so many memories of my daughters' childhood. I think it would be a great book for discussing gender woo with children.