Game concept
Student
The game also links it as Students, Student's, Students'.
From the game's encyclopedia
Students are Characters benefiting from the advice of a Mentor, who is helping them to advance one of their Skills. Mentors are gained through events when your Lifestyle Focus is set to a Skill they excel at.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Student is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Students are Characters benefiting from the advice of a Mentor, who is helping them to advance one of their Skills." It is part of Special Relation. The game also links it as Students, Student's and Students'. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.