I have a couple of things to mention here.
Firstly at schools I have taught in some teachers have taught RI and taken a group of students as they have been trained in their religion and are comfortable teaching it. In all other instances with volunteers there is always a teacher in the room while the volunteers are taking the group. The volunteers are not on "free rein" while teaching about a religion.
It is a tricky path for teachers however as parents have agreed for their kids to be in a class...........hard to judge if what's being taught is accurate for the religion without the personal training (particularly for teachers like me who have not been to church much and have one atheist parent!).
I have supervised a lot of these classes and have to say I found the Catholic Nuns the best at teaching stories of the bible clearly. Also once there was a really cool Rabbi who spoke at a good level for the kids.
I have also supervised the "non-religious" groups at this time......to me it seems such a shame to waste this time that could be used to learn about a variety of religions. Most children in most schools I've been to spend the 1/2 hour doing puzzles or colouring. With such a lot to fit in it just seems a waste of precious time in school.
Secondly I remember when we were kids mum let us choose a different religion to attend each term. I reckon we got a pretty good understanding of the religious stories this way.
Lastly (and I reckon this is funny) my cousins child had a really scientific mind which was strongly encouraged by his atheist dad and agnostic mum. He too was allowed to attend whichever group interested him each term...........he used to ask tricky questions to the volunteers like "how could god build the world in 7 days as even a tree takes longer than that to grow.......and how did god make night and day, was it because he made the world spin? How did god fit EVERY species on the ark and stop them eating each other and pack enough food for them all?"
All these were excellent questions and he got kicked out of two groups for being "disruptive!!!!!"