Select "Courses" from menu.
If the course is on your today's schedule then tap on the course to view its details
else tap on the full-schedule button to select the desired course.
On the Course Details screen, tap on the roster () button
Once you see the roster as shown below tap on the student's name you want to email
If the student's record is not found in your phone's student directory the app will search the student in server as shown below
Once the student record is found the details page from directory shows up as shown below
Finally tap on the email address and your email app will open. (Make sure that you have a working Email app in your phone else it won't do anything.)