SDGK2's gfx editor, while offering many tools is not all in compassing art utility, nor does it try to be. For an included resource it quite amazing and what you need is at your finger tips, not buried away in countless menus. However there is still plenty of room for improvement, as cbass mentioned graphics gale is worth taking a look at.
Anyway scanning images for game art is fine but you wouldn't want to use the scan directly. The problem is that you need to clean the scans. In photoshop you might scan in the image, clean it, set that layer to multiply and then paint on a layer below (for a general art piece). The sgdk2 editor does not currently support layers. This means you would have to do a LOT of clean up, erasing, and tweaking to use the scan layer directly.
Hand drawn/on paper art is a great way to begin the process if that works best for you but SGDK2's editor at current is not the most well suited, though you *could* do it. For game art/sprites what you would typically do is trace the image on a layer above with the pencil tool to get clean line art from which to progress.
My feeling on the guildhall projects is that many of them have traditional art knowledge but a limited experience with digital art tools (isn't photoshop provided in one of the courses?). They are also either in teams or share resources (can't remember which) so there are a number of inconsistencies on the graphics front. I assume that are teaching game design more than game art design at the point the students use SGDK. It's interesting that this came up as I touch on the subject briefly in a piece I'm writing for this section.
You know drawing tablets aren't much more expensive than scanners these days. You can get a nice wacom graphire tablet for 70-80 bucks. I personally have an intous2 tablet but there is no real advantage for sketching purposes aside from available size (my mine is 12x12). Most of the differences between the graphire and intous tablets are realized when digitally painting. More pen sensitivity levels and pen tilt (brush angle) controls. The smaller tablets are more convenient and easily do the job so in less you have the need or money to burn I recommend the graphire. There is a learning curve so it may feel awkward at first but it doesn't take that long get use to it. Once you get past it you'll have many of the benefits of traditional art as well as digital tools in one. At the price point its something to consider even if it's for moderate use (quick concepts,etc). I have used mine for online pictionary =P