OBJECTIVES.: The number of healthy school interventions of unknown quality overwhelms schools. Quality is a construct that is differently interpreted by teachers and health promoters. The schoolBeat checklist for quality assessment of healthy school interventions incorporates the quality perceptions of both professional groups. To support quality improvements - and thus effectiveness - in school health promotion, this study evaluates the schoolBeat checklist. METHODS.: Twenty-nine healthy school interventions were assessed in the Netherlands, each by two health promoters and two teachers-individually and at a consensus meeting. Generalizability coefficients were calculated for the nine specific quality criteria. RESULTS.: The mean consensus score differs from the mean average individual score for two out of nine criteria. To obtain a threshold Generalizability coefficient of 0.70, the number of assessors required per criterion ranges from 1.6 to 10.8, with an average of 4.7. CONCLUSION.: Quality assessment procedures of healthy school interventions using the schoolBeat checklist require about four experienced assessors from each professional domain to facilitate reliable quality scores based on individual assessment only. Publicly available quality scores enable the inclusion of high quality interventions in school policies in order to increase the impact of school health.