How can I ensure that my child won’t meet non-Halachically Jewish children?
You can't. We live in a multi-cultural society and part of the JCoSS ethos is to prepare and educate pupils to understand the meanings and implications of this. As such, if this is a major concern for you, JCoSS may not be the school for you.
But of course, even if you send your child to another Jewish school they may meet non-halachic students and again when they have left school.