The partnership represents a principle of action at the heart of public policies of various kinds. It can be defined as a formal association between organizations that decide to cooperate to achieve a common goal. The formative evaluation of quality conditions of the collaborative action is useful to support partners in improving their practice. This article describes first a tool available to achieve a collective diagnosis by members of a partnership investing voluntarily in an evaluation process and then presents a concrete application of it. It concludes by exposing some knowledge and lessons learned from the experience of the evaluation of a partnership for a comprehensive and integrated cross-sectoral school-community intervention.