If you are sending Email Blasts from the CLASSES page, see our "Sending Email Blasts from the CLASSES Page" document.
The key to emailing future enrollments is the Active Date filter on the left-hand side of the FAMILIES and STUDENTS pages.
Simply change the Active Date to a date after the enrollments begin and use the program, class, and other filters on the page to get the right group of students or families. Then use the view options, checkboxes, and the Email Blast wizard to send out the email.
Note: Another great way to see who was successfully emailed is to check your background tasks for the Email Blast and click the drop down arrow for more information.