Is workman's comp necessary on top of professional liability insurance to teach exercise classes at a community center?

Your Professional Liability insurance covers you for claims of loss made by your students in relation to the quality and competency of your teaching services. It does not provide coverage for bodily injury to a student, yourself nor an employee.

Workman's comp covers you and your employees for bodily injury you may incur in the course of your work. However, the law of States differ on the number of employees that a business must have in order for worker's compensation to be required.

They are two different kinds of coverage, if you have employees you need the workman's comp.