Studies have shown that course participation and academic performance are key factors in defining the success of online education, but much remains unknown regarding how best to define the success of online K-12 after-school education that are popular in Asian countries. To address this issue, we used a longitudinal clustering approach to analyze the course records of a large online education company in China. In total, we analyzed data on 166 online English courses offered by a Chinese K12 after-school education company for the entire fall semester, and after excluding data on 10 classes where there were consecutive missing courses, the remaining 156 classes covered more than 200,000 primary school students enrolled in grades 1-6 in public schools. The results showed that there were two different patterns: classes with poor learning outcomes generally had high participation rates, while classes with good learning outcomes generally had low participation rates. Further analysis revealed that teacher's teaching experience, the difficulty of the course, and students' grade level helped explain the dichotomy. This finding shows that there can be dissociation between participation and achievement at the class level in online K-12 after-school education, which likely resulted from misalignment between requirements set by the course and the expectations from teachers and parents. This study provides important insight for future research and practice in online K-12 after-school education.