Silverlance,
I am not a combat vet but I did train and serve in the Army. Also, I am not a history teacher, but I have a collegiate level history degree. Additionally, I freely admit that my taste in great war films would likely be viewed as "cry-baby pacifist" by many. That being said, I'll offer my comments.
It seems to me, that given your criteria: ex. 90 minutes of viewing time, 9th grade students, a desire to reinforce pedagogical goals (the emotional and expressive aspect of post war writing) and impressing the costs of war, your choices are somewhat limited.
Are 9th grade kids really going to relate to our father's and grandfather's war movies? I think not. I think this rules out the long list of B/W films. Additionally, kids these days are fairly savvy media consumers and they can smell cheese from a long way off. So, in a way, that rules out your more "rah-rah" type war films, you know, the films with wooden heroes who dutifully take that hill. Or the films with characters whose only flaw is that they are young and eager. If the film is more propaganda than war chronicle I wouldn't be shocked if they tuned out.
Next we have foreign films...the students may have trouble relating. Relating to german U-boat crews, relating to German troopers in Stalingrad, relating to Aussie farm boys, etc.
I'm not sure that you are left with much.
A film that I would suggest might be:
When Trumpets Fade. It runs 92 minutes. It was made in 1998 and it is about the Battle of Hurtigen Forest in WWII. It is a gritty film, with real characters rather than heroic caricatures and it depicts themes of sacrifice, redemption, futility, and courage. Trumpets might be a good fit if you just want to get through a whole film which you can then discuss from a writer's perspective.
Another option might be:
A Midnight Clear which is a film based on William Wharton's autobiographical book of the same name. The advantage with this film being that you can actually compare it to the author's work. It runs just under two hours. The drawback is that the film is very slow and really only has one "action" scene.
Now outside of those two, I will echo
Blackhawk Down (though I feel BHD is a bit heavy on the America as noble world policeman theme) and
Band of Brothers as good options and here is why: Both films have books attached and both films have a wealth of additional material featuring first hand interview accounts with soldiers who were present. You could have a list of this material prepared in advance for students and for the two, three or four kids who really get their hackles up, they have ready-made places to go for more knowledge.