If you are SALARY-EXEMPT, that means you are exempt from overtime rules. They can ask for what's in your contract plus "extra duties as assigned" which means, pretty much, whatever they want, especially if there is a stipend agreement for it.
If you are SALARY-NONEXEMPT (for example, most paras) then yes, they are legally entitled to overtime beyond 40 hours. This is federal law, and an ISD cannot ignore it.
This is why many ISDs do not permit a para to be a coach for athletic sports - they would earn overtime hours if they're not exempt.