I'll teach in a 3 month course about the basis of .NET Framework, do you think that the Visual Studio Express Edition (both C# and Web development) are sufficient for learning or I need to buy a complete edition license for each guy involved (actually an EDU edition)?
As long as it's all about the .net framework, and has nothing to do with the IDE, then you could use notepad. I'm sure you'll be able to do anything that you need to w/ the express edition.
I believe the trials for the full version of visual studio is 180 days (3 months) anyway, so you could use the full thing if you wanted to.
You can download the trial versions of visual studio and use it for 90 days.
Finally, i'm sure i shouldn't say this, but you could get VS off a bit torrent site, and i don't think MS would care, because A) you are certifying in their technology and B) it's pretty hard to work in a Microsoft development shop and not be getting their developer tools for free (even MSDN licenses are fairly well priced and you get all the developer tools you need + OS licenses + tons of other stuff).
For teaching the .Net basics, Visual C# Express Edition is good enough. Although, if you are goind to teach them about ASP.NET as well, you'll probably want Visual Web Developer Express Edition as well.
For detailed comparison of the VS 2008 editions, check out http://msdn.microsoft.com/en-us/vsts2008/products/cc149003.aspx
Since you referenced the EDU edition, your students may qualify for Microsoft DreamSpark if they have valid .edu addresses. Free Visual Studio Professional, plus some other nifty things.