Now I'm curious... Did sensei not have the same restrictions when she was training? She did but she broke them? Was she just embarrassed that she drew him, since the drawing itself seemed fine?
@Swift
Actually, Hana-sensei used to trained with the boys before. But she explained that because there are a lot of eloping among students so she and the guy in charge of the boys decided to split the girls and the boys. If you want to know more about this please read Chapter 7 & 8 together (Chapter 8 will give you the explanation you need).
As for the embarrassed part, it is implied that she saw something from the guy she draw, which I won't say too much about it.