Poškolák(1976)

An animal lover, Miloš , brings a small mouse to school, which causes a lot of excitement in the classroom. Miloš remains for punishment after school, however, when the teacher notices that Miloš is bringing lunch to his sick grandmother, she lets him go home. However, Miloš cannot get home with lunch. Somewhere, he lost the key he wears on the string around his neck.