Hi Eric,
One of the possibilities to prevent them from trying to message instead of post, which I used before I retired, was to just turn off messaging. I, and my Principal agreed, did not want to be responsible for for students sending inappropriate messages to each other. My replacement for messaging was the Dialogue activity set so they could only write to me.
You other point of changing the word, Discussion is easily doable, if you are admin or can have an admin do it for you. It can be done via Language customisation.
Go to Administration > Site administration > Language > Language customisation and choose your language. Then click Open language pack for editing. Click Continue after the pack loads. In the, Show strings of these components, scroll down to the mod section, then click on forum.php. In the, Only strings containing box type, Discussion, then click on Show strings. Scroll down the list of results until you find the one for mod_forum discussion Discussion, with an empty text box next to it. In the box, type, "Discussion topic", "My posting" or something clearer. Finally, scroll to the bottom of the page and click, Save changes to the language pack. When asked if you wish to save these changes to your local language pack, click Continue.
You should see your new wording the next time you visit a forum.