Round characters are characters whose motives are relatively complex, and do not necessarily fit a reader's expectations when it comes to their actions. Round characters are very often the hero (or anti-hero) of a work; however, a teacher should make it clear that being round does not necessarily mean a character is the main character. Additionally, part of a teacher's objective with round characters should be to make a distinction between a "round" character and a character that is described in a lot of details. Details do not necessarily make a character round; roundness is found in the character's actions, not their description.
Flat characters stand in contrast to round characters because they are relatively simple. Flat characters are often defined by a handful of consistent attributes. A teacher's objective for flat characters should be to make it clear that flat characters are not necessarily dull and, importantly, they are not just secondary characters. For example, many famous literary heroes are heroes precisely because they are flat: They are always brave and heroic, no mater the situation. Flat is not a negative characteristic; it simply stands in contrast to round.
A static character is a character that does not change through out the course of the work. Like flat characters, teachers should be careful to make it clear that static is not necessarily a negative characteristic, but it just means that the character is consistent from beginning to end. Indeed, the vast majority of characters are static because the work can only focus on so many protagonists. Most secondary characters are characterized statically.
Dynamic characters contrast with static characters, thus, a dynamic character changes throughout the course of a work. Protagonists are normally dynamic characters because they change who they are because of circumstances in the work. These changes must be in their characterization, not just in their material possessions, and they should result in important changes in the character (such as with Pip in Charles Dickens' "Great Expectations," a quintessential dynamic character who changes his personality for the worse when he inherents a large sum of money).