While working & for 3 years after leaving within a 15mile radius holds up in Wisconsin - it's been tested!
I always have my teachers sign one & let them know it will stand up in court. I also let them know that while they working at my studio, they are welcome to come & ask me if they want to do temporary work or have other opportunities. Many times I let them and then they so appreciative, they are less likely to leave then. But I always protect myself, I've been burned too many times.