Open classlist for whichever class you need to email
To email All students (or a subset), use the checkboxes next to the names or at the top of the column:
To email all use the checkbox at the top of the column and you will see all students will have a check next to their name, or select the checkbox next to particular students if you just want a subset.
Then click the Envelope button
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Note - Make sure google is your default email