Since the end of the 1990’s, promoting pupils’ wellbeing is one the schools’ missions in France. It is part of a broader extension of the social function of schools that requires, among other things, to “personalise” education and take account of pupils as “whole-persons”. In this context, I started out a PhD research on the uses of pupil’s wellbeing and mental suffering in French secondary education. Early fieldwork analysis revealed that those categories (wellbeing, mental suffering) did not constitute everyday categories to the staff. They were redefining pupils who challenged the school norms (behaviour, attendance, work) under the prism of “mental suffering” situations. In the organisation’s everyday life, those deviant pupils are qualified by school staff as “pupil’s cases”. “Pupil’s cases” require relational work (with pupils, their family, school staff and sometimes other professionals outside of the school) in order to find solutions to the problem(s) the pupils bring in school. Managing pupil’s cases is often seen by school staff as a way to defend and protect the “best interests” of the child. To that extent, this work fits in the frame of pupils’ wellbeing. But how are those “best interests” defined? This communication is based on qualitative investigation (ethnographic observations and interviews) lead in three French public high schools between 2014 and 2016 as part of an ongoing PhD research. It demonstrates the ways in which the “best interests” of the child are most of the time qualified by the school staff themselves, it can be analysed as a form of expertise on pupils more than a way to give voice to the pupils. It sheds a light on the power relationships between professional territories that are at stake behind the defence of pupils’ wellbeing.