Only approximately half of children and young people (CYP) with mental health difficulties access mental health services in England, with under-identification of need as a contributing factor. Schools may be an ideal setting for identifying mental health difficulties in CYP, but uncertainty remains about the processes by which these needs can best be identified and addressed. In this study, we conducted a two-round, three-panel Delphi study with parents, school staff, mental health practitioners, and researchers to inform the development of a program to identify mental health difficulties in primary schools. We aimed to assess and build consensus regarding (a) the aims of such a program, (b) identification model preferences, (c) key features of the identification model, and (d) key features of the implementation model. A total of 54 and 42 participants completed the Round 1 and 2 questionnaires, respectively. In general, responses indicated that all three panels supported the idea of school-based identification of mental health difficulties. Overall, 53 of a possible 99 items met the criteria for inclusion as program core components. Five main priorities emerged, including that (a) the program should identify children experiencing mental health difficulties across the continuum of severity, as well as children exposed to adversity, who are at greater risk of mental health difficulties; (b) the program should train staff and educate pupils about mental health in parallel; (c) parental consent should be obtained on an opt-out basis; (d) the program must include clear mechanisms for connecting identified pupils to care and support; and (e) to maximize implementation success, the program needs to lie within a school culture that values mental health and wellbeing. In highlighting these priorities, our study provides needed stakeholder consensus to guide further development and evaluation of mental health interventions within schools.