I've found that the best movie to watch in high school is definitely REMEMBER THE TITANS (2000).
Not only is it a really entertaining movie, but I've found that it helps teach students about America but in an un-boring way. The movie a) Portrays an important time in American history b) Shows American football/school spirit c) Shows race relations in 1970's America... but all in a feel-good, Disney sort of way.
I've never been one to have students take notes/answer questions throughout the movie (I hated when teachers made me do this in school) so afterwards, I usually just have a discussion time with the students. I'll also try and reference certain scenes in the movie to drive home the point.
I torrented the movie (shh) and found subtitles off the GOM Player website. I know some people may feel that showing a movie in class may be a waste of time, but as long as you preface that there will be a discussion after the movie are serious about trying to explain some of the themes in the movie, then I think it can be a really worthwhile lesson. It took two 50-minute periods, but you could probably show a clip of the movie and have a discussion afterwards if you wanted to keep it to one class period.
My students really loved the movie--thought it was hilarious and entertaining. Please feel free to reply to this and mention any other movies that are good to show in class!