Teachers’ implementation calendar
’ illness
For each school day between T1 and T2, IG teachers wrote down the exercises they had performed with their class in an implementation calendar. Because the exact number of intervention days varied between classrooms, the number of maximally possible Breathing Breaks (3 * number of intervention days) varied between 148 and 164. When calculating the proportion of actually delivered Breathing Breaks (between 65 and 149) out of the maximally possible Breathing Breaks for each classroom, it was established that teachers led 41%, 48%, 77%, 84, and 91% of all possible Breathing Breaks in IG classrooms. The breathing exercises were not performed because of teachers’ illness, project days, or a COVID-19 quarantine of the whole classroom.