This petition was rejected

The Government e-Petitions Team gave the following reason:

Faith schools are mostly run like other state schools. They have to follow the national curriculum except for religious studies, where they are free to only teach about their own religion.

You could start a new petition calling for Christianity to be added to the national curriculum, so that it has to be taught in all schools. Alternatively, you could ask for all faith schools to be required to teach about other religions as well as their own.