Abstract: This research has three main objectives which are; to study the external quality assessment; to group educational institutions classified by the result of external quality assessment; to compare the change of Ordinary National Educational Testing scores between the school groups which have different the third-round external quality assessment score for basic education in Mathematics, Science, Social Studies, English and Thai language subjects. Research data was a secondary data including the third round of external quality assessment results and the Ordinary National Educational Testing scores (O-NET) from 2011-2015. The research data was analyzed using descriptive statistics, latent profile analysis: LPA and mixed-model ANOVA. The findings were the assessment results: the educational institutions quality assessment result was higher than 80%, except for the learning achievement indicator that had the least value. The educational institutions were classified into four groups; urgent support needed group (4.90%), internal quality assurance emphasizing group (11.00%), quality group (71.30%) and identity emphasizing group (12.70%). The educational institutions in quality group had the level of O-NET scores development better than other groups.