In order to afford moving beyond the standard email or online chat mode of "type & send" (that is, making an assumption that the discourse is okay unedited, that editing's not necessary), a good discussion board will offer -- or, even, insist upon -- a stage of previewing before sending.

Ideally, the preview will (as in HyperNews or Ceilidh) be presented in a different font (possibly less drafty and more formal -- Times New Roman vs. courier, for instance) and display, to invite the student to "re-see" her posting from a reader's point of view.