What's going to happen, enki, is that stock phrases will be taught to children to include in their writing to boost their SATs marks - in this day of Ofsted inspections every mark counts and schools don't have the time to go outside the box. Creative thinking and writing will go out of the window, on this at least. And so children taught this way will be on the back foot as they progress through school and thinking creatively becomes exactly what is expected of them.
I agree with you in part, Rhi, but this isn't exactly new. In all sorts of ways, once SATs were introduced, schools were increasingly tempted to teach with these in mind.
However the subject of creative writing, as distinct from,say, informative writing is an interesting one. To write creatively and imaginatively is very important, but without some sort of a structure to build on, it can easily become uncontrolled and even incoherent to the reader. The ability to produce music, for instance, can be an intensely creative activity, but needs some sort of basic structure to convey that creativity.
So, generally, if a pupil is putting down his/her thoughts ignorant of sentence structure(or even not using sentences at all), the effect to the reader can become quickly incomprehensible, and, surely, the main purpose of writing is for it to be read.
However, I do agree, that if one rigidly sticks to grammatical rules, then this could genuinely curtail creative writing. Freedom, but not total freedom, is probably the way I would put it.