Ideas Needed: Web-based video creation website

I'd like to have my middle school students try their hand at video creation, but we only have one mac in the entire school, and would prefer students use a web-based program that they can access anywhere - in any computer lab, at home, etc. I would prefer to avoid downloading software, if possible (though Windows Photostory and Windows Movie Maker are both good), to increase universal access to my students.

Does anyone know of any free web-based sites that offer the ability to upload pictures and videos, cut, rearrange, remix, etc? My biggest lead so far is http://www.kaltura.com, which seems promising, but I don't know if it has the ability to add voice.

Thoughts and ideas? Even just a website or idea would be appreciated! Thanks.