Laura,
Everything in Moodle is possible. ;) For specific courses, have the teacher edit (update) the course news forum, and then choose the "force subscribe" option from the pull-down menu. Everything posted in that forum will be sent to everyone. You might want to make it so that students cannot reply to a news item.
In the course settings, you can also choose to have course news items appear in a window (you can even choose how many past items to automatically show). If you don't want to "force subscribe" students, you can at least encourage them to check that window frequently.
2. If you turn editing on from the main page (the admin might need to do this), you can create a news forum that appears like moodle.org. Depending on how you want to use it, the forum could be "force subscribed" to all users, but this might be overkill.
-Tom
Teaching with Moodle
Question about making teacher announcement.
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