Can a CA employer enforce a non-compete clause with a contractor while still employed? Yoga studio says PT teacher can't take shifts at another studio

I know post-employment non-compete clauses are generally not valid in CA, but what about DURING employment? My wife works PT as a contractor for a yoga studio in Novato, CA, and that studio had her sign an agreement not to work for any other yoga studios in our city limits while employed. A new studio is opening up in town and they offered my wife a few classes to teach. She will not be giving up her current schedule at the first studio, nor does she market her classes herself (the studios do that) so there's no direct poaching.. Can the first studio enforce this clause? Or can my wife take these additional shifts at another studio?