It wants english medium as the correct answer rather than just english. There is another one for Welsh which allowed the answer with or without the word medium.
Thanks for the comment. Although the general understanding of this phrase in Wales would be an English medium primary school, this phrase doesn't actually have the word 'medium' in it and so the more correct translation would be simply 'English Primary School'
That's now been made the main translation, so thanks again for pointing it out.