In 2020, the global pandemic of COVID-19 significantly affected most human activities, including education. Many countries took anti-epidemic measures to reduce or eliminate the pandemic, which often included reduction or cancellation of full-time face-to-face school education. This, of course, went hand in hand with a number of technical, organizational, and pedagogical-didactic issues that had to be solved instantly. The presented study analyzes the behavior of more than 1000 secondary school teachers in the e-learning system named Techambition for mathematics education developed by the company with the same name. In the study, data from the spring and autumn 2020 waves of the pandemic are analyzed and presented (the period from 1 March 2020 until 18 March 2021). The main outcome of the study is a description of the basic types of approaches to teaching, based on cluster analysis of the collected data. The collected data show three basic types of teachers' behavior during spring 2020 that were identified based on their characteristics: organized, hardworking, and seekers and three basic types of teachers' behavior during autumn 2020: organized, seekers, and experimenters. As part of the findings, we present an interpretation of the data through the description of individual groups and monitor the changes in teachers' behavior between the spring and the autumn 2020 period of the pandemic.