  The air quality in classrooms across all types of schools in Lower Saxony in
the north of Germany has been monitored during June to December 2021 in a large
study. Novel sensor instruments for air quality monitoring were wall mounted in
329 rooms in 50 schools and day nurseries. Each sensor instrument records data
for carbon dioxide CO$_2$, sound level, temperature, and humidity. All
collected data are transmitted for monitoring and further statistical
evaluation by WiFi connection to the database. This study provides for the
first time a detailed survey of the air quality situation and handling in
schools during more than six months. The situation can be characterized by
large variety in classroom area and volume combined with very diverse air
ventilation possibilities. To control virus transmission via aerosol in
classrooms an individual monitoring of air quality combined with high
compliance for ventilation according to the signaled recommendation is urgently
required.
