This is not the assignment. You're trying too hard to brag. I hate to break it to you, but you haven't even demonstrated skilled terraining in that picture, you just used good-looking imports.
The assignment was to create mountains with the height tool, and it looks like, except for the water, your whole terrain is flat.
Check out the back, yo.
y assignment
Anyways, aside from apparently not following the predetermined bull that your teacher
wanted you to do, this is quite nice. However, room for improvement lies in covering up the ugly transition between your regular grass tiles and your reflection tile. You can create an easier transition by using really any kind of base filler
(i.e. grass, bushes, rocks, small trees, or any combination of those - keep in mind that that space can also be filled with fiddily little doodads like fences and smaller constructions so that the background can still be easily seen.) In other words, you want to maintain a certain level of progression in the overall composition, either where things close to your camera are large and things farther away are small
(still want to have an impressive background, but still keep in mind how figures shrink due to distance) or, things close to the camera are small and get larger as the terrain extends
(this technique is primarily used for small scenes that do not require a background other than large objects that meld with a thick fog, for example. Trees bundled into a forest, or a grouping of buildings; those kinds of things.)
What this all means for you, is that you want to create a focal point for your terrain and proportion the things in your terrain so that that focal point is expressed in all its detailed or otherwise impressive glory.
As for the general technique, you want to clean things up a bit with just a few simple actions.
The sky can be made much more cohesive with both your fog and your underlying reflection by using two simple doodads. Stormcloud.mdl and Uber_cloud.mdl That shit is a life saver when it comes to making decent looking skies. Try them out and see what you can do.
The reflection transition, as I said, is pretty easy to do by just adding more work into it in the form of MORE doodads. Fill all that in with bushes and grasses and rocks, making sure to add depth and intrigue into it by layering the different doodads in such a way that their shape and color both compliment and contrast with each other.
As for focal pointing, make your forests large on the peripheral of your screenshot and then have them become gradually smaller as they come out of the forest. This technique represents how forests naturally have their new growth on their perimeter - for a settled area, this can also represent how humans interact with nature by cutting trees down and then nature coming back with similar new growth and begin encroaching back onto the land they once had.
Anyways, I ramble like a motherfucker, so you can read all that if you want, but I don't expect you to stay with me through it all.
Good luck and keep at it, you did just fine.