Religion is an inextricable part of politics, social studies and history.
In so far as politics, social studies and history should be a mandatory part of any secondary education, so teaching about aspects of religion relevant to those studies should be as well.
But that is as different from religious indoctrination as criminology is different from extolling the joys of crime.
Whether religion should be studied would depend on whether the study has more similarity to criminology, or to the extolling of the joys of committing crime.
In so far as politics, social studies and history should be a mandatory part of any secondary education, so teaching about aspects of religion relevant to those studies should be as well.
But that is as different from religious indoctrination as criminology is different from extolling the joys of crime.
Whether religion should be studied would depend on whether the study has more similarity to criminology, or to the extolling of the joys of committing crime.