Objective examination of religions, their relationship with human psyche and their role in history, cannot be confined exclusively to those professing faith in deities.
The uniquely human propensity for faithfully investing actions in service to a greater or 'higher purpose' transcends theology while still definitively qualifying as religion.
Excluding many faiths, labeled secular by self-interested theologians limits the logical scope of the topic, making it more susceptible to bias. Most of human history occurred as it did because indoctrinated populations were sent forth to act on shared beliefs. Some contrived doctrines were attributed to deities. Others were products of leaders' lust for power, comprised of articles of faith in narcissistic concepts like nationalism or racial destiny.
Treating the subject popularly called religion separate from equally devout secular faith movements unfairly allows theologians to portray themselves as superior because of mandates with alleged divine sources. It is astonishing that with straight faces, clergy routinely wave off responsibility for their own barbarisms with one hand, while wagging the finger of the other hand at secular tyrants for the same destructive acts. They get away with attributing murder and mayhem to misguided or apostate associates while presenting themselves as the source of all that is good.
Would you cut slack to neo-nazis for disowning the acts of Eichman or Himmler with the excuse of over militancy or misguided National Socialism? Radical Muslims blow themselves up. Misguided Christians assassinate Physicians. "They aren't real examples of Islamic and Christian love and tolerance like us!" Exactly that kind of counterfeit nonsense and more that would have to be included in order to teach about religions in a public school.
Religions cannot be treated objectively as a human phenomenon or as an historic influence unless open discussion, reasoning and comparisons take place as they do in other subjects. This is why religious zealots will always oppose, meddle in, disrupt or interfere with efforts to teach children about the world's past and present 'faiths.'.
Organized theologies promote the antithesis of individual reasoning; of the freedom of the individual to compare and share ideas outside dogmatic arenas, away from thought containment. Major religions rely on the individual abdicating varying degrees of intellectual autonomy in accepting doctrines and in return for membership. This is why children are started with indoctrination as soon as possible, before reasoning faculties have a chance to develop.
Churches and their minions will never peacefully tolerate their articles of faith being compared with others in the classroom. Discussion of individual and mass behaviors under the influence of religion and evaluating merits of theological claims will not happen unless 'people of faith' can be convinced that Hell has been recently covered by a sheet of ice.
Islam and Christianity have roots in Judaism, have large popular support and for those reasons alone deserve a large share of attention. Faiths like Democracy, Communism and National Socialism are of more recent vintage but also deserve inclusion. Some other more obscure, worship and devotion to symbols includes systems labeled as cult or occult and there is some generous historic overlapping as well. One example of overlap was the superimposition of the Pagan Pantheon structure of minor specialty gods, vestal virgins and Pontificus Maximus on the early Roman Church. New names were inaugurated like patron saints, nuns and Pope but the basic structure survived.
If teaching about religions cannot be all inclusive, free of influence by pressure groups, teaching it in public schools shouldn't be allowed at all.
Institutions of higher education can inform the curious at a time that is probably more appropriate anyway. By the time fertile young minds encounter college, personal experience has revealed that once firmly believed in Santa Claus, The Easter Bunny,The Tooth Fairy and The Stork were all fictional. It stands to reason that they'll be more disposed to examine theirs' and others' articles of faith and learn about the often bizarre roles that faith in higher purposes plays in human events.