This video shows you how to use your onCampus Roster to email a class of students. The Roster can also be used to email the parents of the students in your class.
To set Gmail to be your default email client, go to gmail and click on the "double interlocking diamonds" icon you should see in your URL bar and choose "Allow" and done.
Then, go into Chrome Settings.
Click Privacy and security and Site Settings
Under Permissions, expand Additional permissions and choose Protocol handlers
Click the options (3 dots) for mail.google.com and select "Set as default"
You can then follow the steps in the video above (starting about 0:49 seconds in) to email your class.