When selecting all students or multiple students, you will then click the bulk action icon in the bottom right of the screen.
From the bulk action menu, select 'Send Message'.
Now a compose message screen displays. You can select any associated recipients . Note, School Counselors are specifically assigned to students and School Coordinators see all students at the school. It is probably wise to select the School Coordinator in the event your student does not have a school counselor assigned to him/her. Change the type to Email if you want this message to also go to the user's email rather than staying internal to Maestro . You can attach a file . Select a subject . And lastly, write your message to one or several people. 
A few options / ideas: You can deselect all users except 'Myself' and keep 'Store Message as Student Communication' to make a log of communication. Perhaps to track a DBA or Welcome Call. Also, you can deselect the student and keep others selected if you want to email a parent or school counselor directly without the student being involved. To CC any others, just click the CC button and enter the specific email you'd like to include.