SonicFit is a website where your singers can develop their musicianship. The lessons are organized in a comprehensive curriculum with quizzes to check for understanding and exercises to develop fluency with both written and aural skills. Users can also select specific exercises to drill and change settings to match their needs. All of the material itself is already free.
There is an optional component where instructors can set up classes, assign students work, view scores, etc. To receive this component for free, just email and mention this ad. You will receive the instructor account for free with unlimited free student registrations for the remainder of Spring 2016. (Good for new instructor accounts only.)
If you decide to purchase after your free trial, average anual subscriptions starts at $150 to register up to 75 singers.