What is that we want children to get out of school? Broadly speaking we want children to gain the skills to be able to command their own future in terms of opportunities presented to them in the future. A large part of the necessary skills gained are 'soft' skills such as communication and interaction with other people learned in everyday school life. Religions classes, as well as providing a cultural context to understanding people from all around the world, exposes children to, for many, the first formal rules or guides to morality they will encounter - adding a formal but also meta- element to 'soft' skills. Here comes to my, albeit short point: In deciding whether religion should be taught we should take time to appreciate why it is useful to study the subject.
Personally, and with little authority on child education I should add, understanding moral frameworks that exist in the world is essential for trust to be formed with people or groups who are not from your own moral framework (even if you don't think you have one, you do!), and is fundamental for efficient economies. However while I think religious education used to be the proper way to approach the study of moral frameworks or ideas, now it is dated, if not simply my the fact that it is more often than not not relevant to children today and so is simply unengaged: so next to useless.
My conception of a replacement class would equip students to recognize different moral frameworks, emphasizing how beliefs are formed and supported in different contexts, and stronger emphasis human biases associated with beliefs and talking about more general moral questions. In layman's, perhaps, how to understand other people (and their view points). This was the discourse I believe religion studies tried to produce, we must reform the approach to reach the same goal.