This whole system was aimed at minimizing the infection rates between students which you won't solve by just opening the windows. Imagine you have some smoker indoors: How do you transport off that cancer inducing poisonous cloud without distributing it in the whole place?
Plus in Germany there are quite a few schools located directly at heavily used streets, so just opening windows will not exactly give you really fresh air (I mean, it will lower the CO2, but otherwise its going from one bad state into another), plus it increases the noise levels considerably.
And yes, ACs are still uncommon in Germany. In return, most buildings, including suburb style homes, are not just made from cardboard. This means the buildings have a lot of thermal inertia which can be used to manage heat during hotter phases.
Plus in Germany there are quite a few schools located directly at heavily used streets, so just opening windows will not exactly give you really fresh air (I mean, it will lower the CO2, but otherwise its going from one bad state into another), plus it increases the noise levels considerably.
And yes, ACs are still uncommon in Germany. In return, most buildings, including suburb style homes, are not just made from cardboard. This means the buildings have a lot of thermal inertia which can be used to manage heat during hotter phases.