We're going through almost exactly the same thing with my son - he did Reception, Yr 1, 2 and half of 3 in the UK (across 3 schools due to job moves), then we moved to Switzerland and he went to a French-speaking school for the next 4 years. He also just studied grammar and comprehension, and wasn't ONCE given a blank sheet of paper and asked to actually write anything from scratch.
He's now 12, and 6 months ago we moved to the US and he went back into English-language schooling (he's 6th grade here; would be Yr 7 in the UK). His writing is also to my mind very immature - lots of single clause sentences in a repetitive style, 'I went to the swimming pool. I met my friends there. I had a good time' - but he oddly gets quite good grades; the benchmark must be a lot lower here!
I'm finding this series of textbooks extremely helpful: www.amazon.com/Writing-Grade-7-Spectrum/dp/0769652875. He worked through the 5th grade one before we moved, to give him at least a fighting chance of constructing a paragraph, and I've already got the 7th grade one listed above put by for him to work through during the summer - well, they DO get 10 weeks here, he'll have plenty of time!
Obviously they're aimed at an American audience, but the core writing skills covered are universal, are broken down in the right way to make the child think things through, and it's a good, cheap, entirely self-contained package. The 7th Grade one covers writing basics like main idea plus supporting details, the process of notes, draft, edit, revise, publish final version (I have a lot of trouble getting my son to go back over his own work and amend/improve it), considering your audience, how to structure paragraphs, etc.
Then it goes through different types of writing: creative stories, descriptive writing (my son really needs this one, to encourage him to use adjectives), persuasive writing, informational and instructive writing, and so on.
It's very step by step and instructional, and more useful at this stage I think than essay prompts or story starters, which are good for practice but no use if the child doesn't have the skills in the first place. This way, my son gets lots of practice writing, but is only expected to work on one thing at a time - more adjectives in descriptive writing, considering his audience in persuasive writing, conjunctions in cause-and-effect writing, and so on.
I'm sure it'll all come good in the end. In my son's case, his teacher doesn't seem overly concerned, so perhaps my expectations are a lot higher than they need to be for this age/ school system. And his schooling did at least make yesterday's homework essay 'My Elementary School Experience' interesting despite his immature style!