The war film might seem to be a rich source of material for the study of genre and there are opportunities for identifying features like cliche, the unification of diverse groups in a common cause as a recurrent theme, special effects, and minimalist characterisation. But, more rewarding would be the use of the genre as a way to introduce ideas about representation.
War films reflect polarisations of international relationships, and almost inevitably embody the values and attitudes of the side that made them. In Which We Serve is a fine example, with other gems such as Ice Cold in Alex and The Dambusters.
When a nation is uneasy about its role in a war, its films become ambivalent, possibly even cynical, about war in general. Think about US films during the Vietnam War. Setting aside propaganda such as The Green Berets, we have Catch-22 and M*A*S*H are epics of disillusion.
What about "realism"? It doesn't matter how well researched, how detailed depictions of combat may be, the reality that war films deal with is emotional. Relationships and the way they develop or crumble in extreme situations - that's the significant content. No war film that I can think of is reliable as a source of social, military or political background material - as documentaries, they are mostly non-starters. Even Das Boot has been heavily edited for dramatic effect. Take any war film (with the possible exception of the suspense movie, Memphis Belle) and ask your pupils to examine and account for the representation of "our boys" and of the enemy. Ask them also to examine representations of women in any war film - even Top Gun.