The contents of what you're trying to teach make all the difference.
Are your tests very visual requiring interactivity with visual objects and media or are you just asking questions with various form-esque inputs (text inputs, radio inputs, etc)?
If it's very visual then it's worth considering AIR to some small extent but you'd really need to have a lot of time invested for that.
Ultimately I'd be converting it to HTML5/JS/CSS3. That's the 99.9% positivity rated presumption I'll make. As a general consideration, where Flash may previously have been used to "grade" a paper or tell the user their current answer is correct, this might need to change. You should be sending the users answer to a server to get a response. JavaScript is plain-text and can be read so you'd want to curb cheating by making sure the server contains the answers and not the client. This is an automation you'll lose by abandoning SWF.
I use C#.NET myself and I'd avoid silverlight like the plague. Let's be very clear here. Flash is not going away. They are repositioning themselves to handle more sophisticated endeavors in the future like full 3d games. Flash is auto-installed with the OS, the user base is not getting smaller. Silverlight almost never had a purpose to begin with, besides Microsoft just wanted a sandbox to play in itself. Windows Metro is the only potential obstacle in the future as it has been said to load SWF content externally, like an application, rather than embedded into the browser.
