Your students can create an account on Amara.org, join the Captions Requested team. Then in its "Videos" subpage, they can click on the "Add Video" tab to add the video they want subtitled to the team.
Team members will get an email notification that there are new video/s ready for subtitling.As there are over 3'200 team members, often the video gets subtitled, but not always. So your students could also share the link to the relevant Amara pages via social networks, asking for their friends' collaboration.
However, in my experience as a foreign language teacher, I've found that students often really enjoy making the subtitles themselves, collaborating in small groups if the video is a bit too long for one single person.