If you are interested in exploring the possibilities of using Twitter in the classroom, you may find these four tutorials by Timo Elliott from SAP helpful. They describe how to install different free Flash plugins which allow you to automatically send and display live tweets in PowerPoint as well as conduct online surveys to poll the opinions of your pupils.
For more ideas on how to display live tweets or use Twitter for educational purposes, have a look at this four part presentation by Petra Pointer at the recent IATEFL Conference in Harrogate called What students can get out of Twitter
Just to clarify, I think Twitter is a wonderful CPD tool for teachers, but I wouldn't recommend pupils having their own accounts. Peta is referring to post-16 students in her session and I think that's OK, but I would suggest for school-aged pupils that they use Edmodo instead or a VLE for home-school links to protect their own privacy.