Based on my experience (5 teachers so far) and that of a couple of friends, I would say you'd have less than 1 in 10 chance that your kids' teachers would speak no English. And even the ones who 'speak' no English apparently understand a lot but are just uncomfortable using the language.
The teachers will almost certainly speak enough for communication about day to day issues, and for the more formal parent-teacher conferences they can almost always rustle up a colleague to translate. You'll also certainly find that some of the other parents are fluent English speakers, who are happy to practise/ demonstrate their brilliance, and help out with little ad hoc conversations.
The schools are very, very used to this, and non-French English speakers are in a much better situation than Japanese or Albanian speakers! And learning alongside your child's homework is remarkably effective.
kodokan