understanding the 40 minute session limit ?

Due to the pandemic, my 90 year old mother has started giving her music lessons online with Zoom. She had absolutely no experience with computers or any technology, so I set her up so that she always the same personal meeting ID and gave all of her students the same link which includes this meeting ID it works fine EXCEPT there are two little girls who are sisters who take half hour lessons from the same location, one right after the other. Because of the 40 minute rule, they get kicked out in the middle of the second lesson. I suggested that they end the session and click on the link to come back in. This doesnt seem to work. Sometimes they get a message that my mom is in another meeting ( this shouldnt happen because she can admit more than one person at a time) Other times nothing happens at all. I can't actually see what they are doing because they are at another locotation How can I trouble shoot this remotely? What should I be telling them to do? thanks