I believe it's important to make a clear distinction between religious education and religious instruction.
For clarity, I'm focusing my remarks as they pertain to public schools, grades 1 through 12.
I see nothing wrong or even illegal in discussing religion in historical terms as it relates to the development of societies. It would be difficult to teach history of Medieval Europe and not discuss the role of the RCC.
One can certainly look into the differences in belief and practices between the RCC and the various protestant reformers of the 16th century to the present, for example, without leaning toward one side or the other--if one is skillful.
Religious instruction, however, is the attempt to convert or otherwise lead a person, or persons, to believe in a particular religion. This is absolutely illegal in the US public school system.