Hi,
I can confirm that saving the canvas to a PNG file doesn't work on Windows. It works fine on Linux (the platform we as developers mainly test on). I have an idea what the bug might be, but I'm not completely sure. I'll see if I can get a development environment up on Windows and have a look at it.
It's really cool to read about how KTurtle is being used. If you or any of you're students have any suggestions of how to make KTurtle even better, we're looking forward to them.
As a workaround to not being able to save or print the canvas, you could make a screenshot of it and print or save that. In Windows XP you can do this by pressing Alt + PrintScreen on your keyboard, then open, for example, Paint and paste the contents of the clipboard. You should then have the contents of the KTurtle window in Paint. Use the selection tool to cut out the canvas and save or print that. Windows Vista has a special tool for making screenshots, you should be about to find that somewhere in the start menu.
We're working on, and have done a lot already, to make KTurtle in KDE 4.4 have a lot more features when it comes to exporting the canvas.
Regards,
Niels