So WHAT SHOULD teachers do?

Let's make a positive statement here, building on earlier commments that I've made.

First, evolution is science - important and well-established science that is foundational for biology and all the related disciplines. Children absolutely should be taught the basics of science at school. Hence, I am absolutely saying that evolution SHOULD be taught in state schools. Whether or not there are constitutional issues in the country concerned, as in the US, science should be taught in what I call a religion-blind way. I.e. there is no reason for a teacher in a science class to mention that creationism even exists. Creationism is religion, and does not belong in a science class. Evolution is science, and it definitely DOES belong in a science class.

There are, indeed, genuine scientific controversies within evolutionary biology, such as the precise importance of genetic drift as a mechanism for phenotypical change; the role, if any, of group selection; and the likelihood, or otherwise, that certain human behavioural propensities are genetically coded (in the sense that the same propensities will tend to appear in a very wide range of accessible environments). But there is NO genuine controversy about whether the main propositions of contemporary evolutionary theory are true. Sufficiently advanced students (perhaps in senior high school) should be taught about genuine controversies at the cutting edge of science; they should not be taught about sham controversies that have been concocted for the purpose of undermining mainstream science.

There is no more reason to teach about a fantasised "controversy" between biological evolution and some alternative than there is to teach about an equally fantasised "controversy" between the approximately-spherical Earth theory and the flat Earth theory, or a "controversy" about the oxidation theory of fire and the phlogiston theory of fire (or a theory of fire that involves combat among angry demons, if it comes to that).

Hence, in a biology class, evolution should be taught on its own merits with no reference at all to religious doctrines or religiously-inspired pseudoscience. If these are brought up by students, teachers should explain politely that they are not science and have no scientific support. The same applies to any other body of scientific findings - e.g. in geology, or astronomy - that contradicts various religious beliefs. Just teach the science on its merits.

In a history class or a literature class, or philosophy class, a wide range of religious ideas may need to be discussed because of their historical influence or literary treatment, or because they have been the subject of philosophical reflection and examination.

When they are discussed, there's no reason for teachers to go out of their way to attack religious doctrines, whether it's the Genesis account of creation; or the Catholic doctrine of transubstantiation; or the supposed finality of the Koran; or idea of achieving moksa through meditation and detached conformity with your dharma; or the wisdom of sacrificing cattle to avoid being struck down by Zeus's thunderbolts. Where necessary (perhaps in a history class), discuss these ideas politely as things that some people believe or have believed, but don't give any of them special credence. Phrases such as "superstitious nonsense" should be avoided, but the teacher need not pretend to think that all these doctrines are somehow true.

Religions and religious ideas should be discussed with neither the teacher's endorsement nor the teacher's disparagement - but of course there has to be a margin for pedagogical discretion in such classes, or teaching such subjects will be impossible. E.g., some religions have been harsh in their social effects, and teachers cannot be expected to pretend otherwise when discussing a relevant period in history when this became apparent, or a literary text in which religion's effects are dramatised in some way.

Finally, if I had my way, comparative religion would be a subject in schools for students from a young age. Students should learn accurate information about what religious beliefs have been held by people in the past, and about the variety of beliefs that are held now. They should be taught about all these without any of the beliefs being advocated or denigrated, just to convey the great variety of what has been on offer, and to give students the kind of cultural insight that comes from being knowledgeable about what a wide range of people have believed about the world and humanity's place in it.