No school can possibly teach about all religions: how can schools fairly choose which religions to teach? If that curriculum is set at the state level, the state is basically endorsing some religions over others and acting in a way that could further recruitment with taxpayer money.
When I took philosophy classes at community college (taxpayer funded), a portion of that class was comparative religion.
It CAN be taught in an objective manner provided that there's a good teacher who teaches it objectively.
