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I am a licensed intellectual property attorney with over a dozen years business handling matters like this one. It is a pleasure to assist you today.
It is perfectly legal to have such an arrangement and you are not creating an employer/employee relationship. No different that reaching an agreement on a commission basis, this should be handled by written contract. You need to create a simple contract wherein your terms are specified.
As a bit of advice, since you are wanting to use their specific skill sets to teach these classes on their techniques, you need language in the agreement which requires specific performance and which notes that their requirement under the agreement is not assignable to another artist or entity. This will prevent them from crawfishing and trying to get someone else to teach the class for them.
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