I would make it part of the instructors procedure to have class attendees sign into class. A simple clip board and pen are all it takes. If you have interns working at your club, they can do the tallying. If you want the instructors to do it, you will need to pay them a little extra. If you do it, do it everyday or it will become overwhelming. Though if you number the sign in sheet, you only need to look at the number of the last person to sign in.
There are other ways to do it. Card swipe systems, front desk check in, etc.