To email classes using org tools:
In the side menu, click Org Tools > Members.
On the top of the page, click Customize Filters...
Use the filters to find the desired group of members.
Once filters have been set, use the corresponding checkboxes to select some or all of the list of members.
On top of the page, click the Communicate dropdown.
In the dropdown, select Email/SMS Custom Message.
Click the Email checkbox > Create Message.
Complete the email template as desired, when finished click Send Message.