The third International School on Beam dynamics and Accelerator technology (ISBA20) was held for seven days from February 25 to March 3, 2021. The school was shifted from December to the end of the Japanese fiscal year in the hope that the COVID-19 epidemic that raged in the spring of 2020 would die down, but our wish was not answered and the school was brought online. Although we were not able to have the hands-on software training for the students that we had originally planned, we were able to finish the school successfully with lectures and presentations by the students. In this paper, we present an overview of the school and discuss the gains and losses of the online approach and the challenges.