I recently stumbled upon the most fantastic micro-blogging website for teachers and students called Edmodo.  Here is part of the post from the Edmodo blog:

What is Edmodo?
Edmodo is a private microblogging platform developed for use in the
classroom by teachers and students. Edmodo provides teachers and
students the ability to share notes, links, and files to foster
communication inside and outside of the classroom. Teachers also have
the ability to post alerts, assignments, and events to share with their
students.

Who runs Edmodo?
Currently Edmodo is owned, operated, & funded by Jeff O'Hara &
Nic Borg.  Jeff is a network administrator/webmaster for Community Unit
School District 200 in Wheaton, Illinois. He also blogs at http://blog.zemote.com.  Nic is the webmaster/web developer for Kaneland Community Unit School District 302 in Maple Park, Illinois.

 Here is a screenshot of the page that I created for the students in my Business Management course at heritage College.

Edmodo screenshot

I normally use email to to update students on assignments, events
and makeup assignments which can be tedious especially if your teaching
three classes like me.  Now, with Edmodo, I only have to post the
assignment once and Edmodo will send an alert to all of my students! 
The best part about Edmodo is that its FREE, so take advantage now. 
Thanks Jeff and Nic!