Apple is known for rendering their video files in very high quality. You might want to see if students can lower the quality at the time that they export the video. Here is a guide from apple on how to set video resolution in imovie ~ https://help.apple.com/imovie/mac/10.1/#/move6e0cb2ad
If this doesn't work, I would recommend students upload to a service like youtube, set the video to unlisted, and then embed the video using the rich text editor. You might also see if your school has purchased Canvas Studio. Canvas Studio is a video solution offered by Instructure and it can handle large video files.
Another thing to watch out for is if this is for an ungraded discussion, the uploads actually count against the student quota. This quota is usually set pretty low per user.
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