Noun adjuncts were traditionally mostly singular (e.g. "trouser press") except when there were lexical restrictions (e.g. "arms race"), but there is a recent trend towards more use of plural ones. Many of these can also be or were originally interpreted and spelled as plural possessives (e.g. "chemicals' agency", "writers' conference", "Rangers' hockey game"), but they are now often written without the apostrophe, although decisions on when to do so require editorial judgment.
There are morphologic restrictions on the classes of adjunct that can be plural and nonpossessive; irregular plurals are solecistic as nonpossessive adjuncts (for example, "men clothing" or "women magazine" sound improper to fluent speakers).
Regarding this I'm still not sure which set is correct:
- student welfare officer or students welfare officer
- student union or students union
- student loan or students loan
- sport center or sports center
Can you please provide some guidelines?