We are working hard with Google to be able to block access to personal Google Apps accounts on school devices. We are aware that this is a concern for many of our admins. While we work for a permanent solution, we offer this workaround to our admins.
Add the following sites to your custom blacklist:
Please keep in mind that if you have Google.com added to your Whitelist this workaround will not work. Google.com is already hardcoded into the Whitelist, it would be best to only Whitelist specific subdomains instead of Google.com as a whole.
If some students have already added accounts, you may need to clear cookies on the device so that they cannot simply click on ones that have already been added.