On the People page, you have the list of teachers and students in your class.
To email an individual student, click on the 3-dot menu to the right of their name. Then click "Email student".
This opens the message box. Type in the subject, message, and include any attachments, then click Send.
If you want to send an email to all the students, click on the check box next to "Actions" to select all the students, then click "Actions" and "Email". This will open the email message box as before.