What is it that you are going to be using this for? Since you mentioned a 'teacher', i am assuming you are still in school and this will be used in an academic setting of sorts. Do you see yourself working in Photoshop alot? Image editing/touch ups? Illustrator and vector based drawings? Is this just as an alternative to the mouse in an attempt to avoid carpeltunnel? Are you looking to 'Paint' and create digital works of art?
Depending on how you answer you answer that, it will be easier to find the right tablet for you.
Also I think the two main things that come into deciding between tablets.. and some people have already mentioned this are.. Size and Levels of Sensitivity.
Size is pretty straight forward to understand. How this translates into 'usage' thought depends on what you are using the tablet for and how your drawing style is. An example where a larger table is nice.. woudl be if you work/draw with long broad strokes.
Levels of sensitivity refers to the various amounts of pressure that hte table is able to sense. The Intuos series of tablets offers roughly 2x the sensitivty, but you pay roughly 2x the price for it
Me. I use an Intuos 6x11. It serves as my mouse 100% of the time. and maybe 15% of my time is spent in Photoshop and the rest is in Flash.