This study was designed to assess the effect of the quality of School Managers' development on their effective operations in schools. One hundred and twenty (120) school managers were drawn from the four (4) Local Government Areas (LGA5) under Lagos State Education District Ill South West, Nigeria. These include: Epe, lbeju-Lekki, Eti-Osa, and Lagos Island. Thirty (30) School managers were selected from each LGA. T-test statistical analysis was used to analyse the data collected. The two (2) hypotheses formulated were analyzed at 0.05 level of significance. The result indicated that there was a statistically significant difference in the assessment of effective operations between school managers of high-quality development and those of low-quality development. However, there was no significant statistical difference among male and female school managers who were of high-quality development in their effective operations. It was recommended, among others, that continuous high-quality development and follow-up of school managers should be embraced or embarked upon by school supervisors to enhance continuous effective operations.