Somebody mentioned gerunds ? And not gerundives? Surely a distinction every schoolboy knows (well, they do if they do A level Latin).
It's a misguided attempt to stop children being discouraged. Let's face it, even 50 years ago,even able children could be disheartened and give up on the subject if constantly failing. The answer was to encourage them in what they could master in it and go on from there. But that's all right if they're in a Public School or prep school, with the resources for individual teaching for every pupil,if needed, and small classes anyway. Normal schools haven't got that available to them and are reduced to gimmicks like banning red ink, as if having purple or pink, or any other colour, on the work isn't discouraging.