During the pandemic, school and university students had to urgently move away from traditional classrooms to online learning in their homes. Student engagement that can be explained by self-determination theory (SDT) is important in online learning. Indeed, the founders of SDT recently stated that SDT-based future research should look closely at how to satisfy the basic needs in the theory in technology enhanced learning environments. We also know very little about K-12 student engagement in online learning. Therefore, this qualitative study used the SDT as a framework to understand K-12 school student engagement and disengagement in online learning. It used a thematic analysis to analyse interview data from 36 students and 18 teachers. The findings highlight that (i) online learning environments that supported more autonomy were more likely to engage students cognitively in developing two important lifelong skills of digital literacy and self-regulated learning; and (ii) those environments that lacked emotional attachment, equipment and resources, coupled with perceived digital incompetence and ineffective learning experience of the students suppressed cognitive and emotional engagement. Hence, this study suggests how to satisfy the need for competence and relatedness to prepare and implement online learning.