Firstly, a lot of kids find grammar really hard. They cannot understand the difference between a comma and a full stop, no matter how many emergency grammar sessions you run. At some point, you have to cut your losses
But at what point? And why not simply deduct marks for poor written expression (to the point of failure, if necessary)? It didn't kill my generation. I'm actually disappointed that I was not taught more grammar than I was.
Most kids don't care. It's hard to make them really, actually care. If they do care, they will be on it already.
You write this about spelling. In post-school education, I see it in relation to many aspects of written expression. Amongst colleagues, we gnash teeth at the way that these students were passed in school leaving us to deal with the (now entrenched) problems. Perhaps losing marks would force them to care? We get a distinct impression that students are never failed because they ^don't" seem to care. Mind you, universities are caring less about such fripperies these days. Customer satisfaction rules the waves!
I'd love it if marking every spelling in red DID have an effect - and, to be honest, as a reflex, I still circle almost all errors. But I have marked errors on COUNTLESS coursework drafts (back when you were allowed to do such things), only to have the final version handed in with the SAME ERRORS STILL PRESENT. Even when it is GCSE COURSEWORK
Then presumably they are also accepting they will have marks deducted for poor writing and this will affect their overall mark?