The full source code to DNN is available at the site. As to documenting it for a school project I guess I do not understand. It would be an extremely (extremely) advanced class to have the educator ask, "I want you to document DotNetNuke". I do mean extremely advanced. If that is in fact the case then you are apparently in the wrong class at school as you'd not be asking what you are asking.

I've been a professional programmer for 25+ years and have been looking at DNN source code on/off for the last 6 weeks and have a hard time following it/understanding it. I really wish it were heavily documented.

If your task in school was to create an interactive web site using ASP and document your work then there is no way the educator expected you to use a existing framework. There is also no way that any educator be they first year IT or fourth year IT will believe that a student created DNN as a class task, just no way. I can not see any educator saying, "We want you to use DNN and create a site, then document DNN's source code in the applicable places to display how the functionality works". Reason I say that is that I do not know many educators (and I know MANY educators) that would never expect this of students unless they want every student (or group) to fail. DNN is not "lightweight" its a very complex framework which is what makes the modules much easier to code up.