"Woman students" OR "Women students" - which one is correct?

The generally accepted rule is that when one noun that is semantically plural qualifies a following noun, whether singular or plural then it is in the singular according to this pattern: it is compulsory if the plural is formed by adding an -s and optional otherwise.

It does not seem to matter in what way the first is qualifying the second - it could be a question of whether the soldiers ARE women or girls, or of whether the fertilizer is FOR cacti or orchids.

The attached pictures show how we can use singular and plural together where any logic would say we should use one or the other.

This explains why there is little argument on this site about girl/girls, but there is lots of argument about woman/women. Because it is optional, there will always be some who say one or the other is correct, and some situations where one or the other is more common.

I have seen this rule in a big old English grammar but I regret I do not have the reference at present.

I have never seen an explanation, but the one that seems obvious to me is that women soldiers is unambiguous, but girls soldiers could be confused with girl's soldiers