I was taught this in High School and I know that it is still taught. We learned it because the option for such (and many other) errors was a zero on the work and likely an F in the class overall.

Teachers have a lot less latitude nowadays; see [1] for an example that is shocking in the 1960s context.

1. https://www.cbc.ca/news/canada/edmonton/teacher-who-gave-zer...