I don't think your idea of cohort is the same as what Germany is doing. Their cohorts number in the 100s.
"Classes have been reorganized, creating so-called "cohorts" groups of several hundred students. The "cohorts" are advised to stay apart, but social distancing rules are being done away with within each group. Classes are being scheduled on a staggered basis. Each cohort has its own area in the school grounds, cloakrooms, restrooms and canteens.
The hope is that if there is an infection, only the respective "cohort" will have to be quarantined, rather than the entire school. In the event of new infections, it is not the schools that decide the next course of action, but the local health authorities. For example, whether to quarantine an entire school or just the respective "cohort."
"Strict statewide hygiene rules have been put in place: Students are told to keep their hands off the banisters when taking the stairs — and to wash them frequently. Disinfectant is to be used sparingly and only when deemed necessary — and mixed into cleaning water rather than sprayed pure. Masks are not obligatory and teachers can avail of free testing for the coronavirus."