Appropriate use of cohesive devices is an important writing skill. Recently, there has been growing research on cohesiveness in ESL students' texts at university level. However, research into secondary school students' writing and learners' perceptions of cohesive devices has remained underexplored. This case study investigated the use of connectives, a particular type of cohesive device, by nine ESL secondary school learners. In this study, qualitative data were collected from classroom observations, students' compositions, and semi-structured group interviews. Techniques of corpus-assisted discourse analysis were applied using "UAM CorpusTool." The findings show that the main problem faced by high-proficiency learners was the overuse of connectives, while average- and low-proficiency learners tended to overuse and misuse them. The findings also reveal that inadequate knowledge of connectives, low awareness of genre, and poor understanding of writing requirements were possible reasons for these problems. Based on the findings, practical implications for language teachers and suggestions for further research are discussed.