High school curriculum doesn't teach Oxford curriculum, and Oxford doesn't teach high school curriculum. This is that situation of "they'll teach you that next year" and next year they start with "we're going to skip this, because you all should have learned it last year".
Well school shouldn’t be teaching you that that is a square. A square is a polygon, and a polygon must be a closed shape made of straight lines (assuming we’re in a euclidean geometry here).
No, I'm in high school, they weren't that specific. I forget the exact definition they gave, but they didn't say it had to be straight lines. I also showed it to the geometry teachers and they said they didn't like it, but based on what they have to teach, it's not wrong.
Bro my elementary school teachers were that specific. We didn’t talk about polygons in geometry. And your teachers shouldn’t say based on what they have to teach. They should tell you that it’s wrong, and not care about if that is a specific thing they’re required to teach. Any research will immediately confirm that the shape you showed is not a square.