To attach a file to a forum post or to access an editing toolbar in a forum reply, select Advanced beneath the response textbox.

In a Reply textbox, you can use standard keyboard shortcuts for simple formatting (bold, italic, and underline), but for any embellishments beyond that (inserting links or images, adding bullets, creating a table, etc.), you need to select the Advanced editor. Once you open this editor, you'll be able to use all of the formatting options available in our normal HTML editor as shown below.

Consider adding this link to a forum description to help your students format their responses: Format Text in a Forum Response.