Technically, comparative discussions about religion are secular. so there shouldn't be much of an issue
Any discussion about religion ought to also include the arguments for why many believe that religions are based on fictitious myths. And if you get a devout Christian teaching the Religious Education class, they're likely to balk at or refuse to present the secular arguments for why some people have rejected religion. Because religion is very emotionally important to people, it is going to be very difficult, in practice, to ensure that biased teachers give students a fair and balanced picture.
