To me needing to have a student to do this just indicates a poor gym culture. If you need a senior to beat up other students then something has gone wrong.
Building the correct team culture is in my opinion the most important job for a coach. This means explaining exactly what to do and not to do in training and sparring. It means that you need to monitor what is actually happening and making sure students are actually following those agreed upon rules. It also means that if someone is repeatedly not following the rules to tell to find somewhere else to train where they will be a better fit.
The lazy approach of not explaining the rules and behavioural expectations of training at the gym and instead leaving it to the mat enforcers to sort out is what always leads to the development of a terrible mat culture and the inevitable decline of a club.
If you’re a BJJ coach it’s your coach to explain and enforce exactly what the students should and shouldn’t be doing when they’re in the gym.
Mat enforcers in BJJ