In my opinion, teaching students to do a large amount of "fiddling" with the ends of lists is getting them used to bad habits. If you _really_ need to do a lot of stuff at the end of a list-like data structure, use a Seq or something.
My tutorial introduced trees and combinators also. ;-) Also, I would point out that even "Real World Haskell" starts with a list.
If you still want to use this to teach students (maybe because you're building them up to "but in general you shouldn't do this") then provide this function for their use elsewhere (either as something to copy/paste, or an auxiliary library for your class). But I don't think it belongs in Data.List.
So, what about 'reverse'? Both 'reverse' and 'tailDropWhile' are inefficient. Data.List has 'reverse'. Why not 'tailDropWhile'? P.S. I don't think students who do not understand lists can understand trees. --Kazu