Originally planned to be produced in 2020 under the name School 2020, and to air in the second half of 2020, the series was delayed for completeness.

[24] Ahn Seo-hyun was cast as female lead, but was removed from the series due to a disagreement between her father (who also served as her talent manager at that time) and the producers.

[29] On July 27, it was reported that Choo Young-woo has been offered the role of transfer student, which originally Kim Young-dae was playing.

[31] On August 5, it was announced by Kurznine Entertainment that Seo Hee-sun would debut as an idol trainee in the series.

[32] On the same day, Mystic Story announced that Kim Kang-min also joined the cast of School 2021.

The production team said, "Through a special broadcast, we will present an exciting time by highlighting a fresh and refreshing story and teen romance once again.