In a sense of what's fair, it depends upon your hourly rate. If you are getting $250 - 350 per hour, then I would say it isn't fair to do extra work. However, in the alternative, there are some places paying $400 - 500 per hour, which may require a little more outside work and preparation. Typically, I've heard people getting paid extra hours for outside work which can offer some benefits if you do it at home or on public transit in half the time, especially if you already have a base of materials to go off of.
Point being, if your boss can get this from you, soon they will ask for more and more until you flip (not always but rarely is there a benefit to doing extra work). Stand your ground. It really sets a poor expectation for those in the field when teachers will work for just one hour, or have two hours with a huge unpaid break between classes, or do these extra tasks. These acts are not sustainable. Centers are a dime a dozen and easy to find those without any of this outside work.
Not saying to quit, but know where your line is.