When I taught English (GCSE, not ESOL) to adults, I had a couple of muslim ladies among my students. It's fair to say that I could understand them very well, so good was their spoken English. The veils were fine enough not to muffle what they said, although facial expressions were impossible to read. As they relaxed in the company of fellow (male) students over the weeks, they began to leave the veils off. No noticeable difference in their speech, but it was a real joy to see them laughing and being 'real' women, communicating to their full potential.
But I agree with Hugh Spencer. I think children need to be able to see facial expressions and mouth shapes in order to learn effective communication skills. My adult students already had a good command of language, but children are still developing in this area. They learn by imitation, so what they're going to learn by imitating a woman who is unable to communicate fully is that women should not communicate fully, as men do.
In an Islamic school, that may be OK. But we are talking about a state school here where although many children are muslim, some are not. If parents wish to provide their children with a state education, then one woman's beliefs should not be allowed to hinder that.