I wonder why the substitutes are nor contracted.
That is, they come to work as independent contractors. They are paid a contractually negotiated fee. The independent contrctor then becomes liable for dealing with witholding and payment of taxes and compliance with Obamacare. They can then work as many hours as available.
A lawyer recently told me that the rules for considering independent contractor status have tightened so it’s more difficult to get by with that than it used to be.
One of the main criteria is whether or not the employee is being supervised and given specific instructions on how to perform the job. That definitely applies to a sub.