Long Drive - S4-E11
Corrected entry: Hal states that Malcolm was supposed to pick Dewey up at school, but the earlier episode "Humilithon" established that Dewey now walks home unsupervised. (Malcolm doesn't yet have a driver's license, so this can't be a matter of convenience.)
Correction: Actually, in the end of "Humilithon," when Hal and Lois finish community service they tell Dewey that he doesn't have to come home alone anymore, and Hal takes the house key from him.