We do use teachers for invigilation duties but there is no easy way to add a teacher to a specific exam and then make them unavailable, thereby creating a cover requirement. Clearly, this would be an optional requirement as the teacher would not necessarily be required for every exam, as we also use external invigilators for exam invigilation.
There are work around but this causes problems with creating dummy accounts in HR Manager that impacts upon other systems within a School's MIS as the dummy teacher will be required to integrate with systems that record statistics of compliance.