We created a little app of our own to replicate your scenario. In the end we have a list of the ‘Teachers’ who replied to the emails against a particular ‘Request’. I believe that is what you wanted.
From the screenshot below you can see I created three basic objects i.e. Teacher, Request and Messages:
Once a Request record is created I link all the teachers against it and send them an email each. Once a teacher replies, a rule is triggered to ‘Add relationship by matching’ as per the screenshot below:
Given the base object of this next pop up page is ‘Request’ object (screenshot below) we can see a list of messages sent as well as teachers who replied.
The crux of the matter is the rule that adds relationship between ‘Request’ object and ‘Messages’ object. The trigger of this rule is Record created against Messages object.
I hope it all makes sense and if you’d like to have further assistance please feel free to drop me a message via Community platform, I’ll be more than happy to assist.
Thanks for posting the attempted paths from your build, it actually helped.