Sounds like a codec problem.
A codec is a coder/decoder which makes a video fit into a certain container (eg avi, wmv, mov etc).
Think of a codec as a mathematical formula that tells a computer the right way to put together all the 1's and 0's of a file into a video.
There are literally thousands of codecs available, hence TeacherTube can not cater to them all.
How to solve your problem(s).
An upload time of 7hours implies a very big file, so we'll try to solve that at the same time.
As you are outputting an avi I'll assume you are using a windows based machine.
I've always had good results from uploading high quality windows media files (.wmv) so we'll try that route.
First off, if the program that you created your video in can do so, try outputting from that as a high quality wmv file (depends on your program as to which options are available).
If you are unable to do that then try downloading and installing the free windows media encoder from
Once installed, you can use this to covert any avi that you can view on that computer to a more universal wmv (version 9) file.
Use one of the standard presets for computer download (above 512k is best).
If you are happy with the result, then try uploading again
An addendum to this reply that I've used before is that you can now get the new version of what is now called the "Windows Live Movie Maker" (if you are running Vista or Windows 7). As well as having hd wmv output). It is a free download from http://download.live.com