An easy way to keep track of your kiddos and their attendance is to get in the habit of pulling an attendance report on each of your classes on the same day each week (such as Mondays so that you have the week to follow up with the students and parents). Here is a screencast on how to pull that report to make your life (slightly) easier.
I had a teacher ask me if there was a way to hide or remove students who are no longer in your class. The answer is maybe. It depends on if they are officially dropped from your class and appearing as red in your roster and gradebook. If not, then no, you cannot remove them until they have been unenrolled from the class. If the student is showing up as red in your gradebook though, you can hide them so you don't have to worry about them. Here is a screenshot to help you do it: