Rhea asks you to speak with the three House Leaders in order to learn more about the three houses. Edelgard can be found in the Gathering Hall while Dimitri and Claude can be found by the Classrooms. Upon speaking to them, they will give a short introduction and allow you to ask in detail about their fellow classmates. You are not required to ask them about all of the students in order to complete the request. Once all three have been spoken to, return to Rhea to complete the quest.
"Speak to the three house leaders and gather information about the students in each house. The students can be found in the reception hall on the first floor and throughout the Officers Academy."
"You should speak with every student you can. This might be your only chance to see what they're really like before you become a professor..."
Quest Completion Dialogue
"You are responsible for choosing which house to lead. If there are still students you have yet to speak with, try visiting each house's homeroom."
Given to you by Lady Rhea located in the second floor of the Monastery