Kenapa tidak berangkat ke sekolah

What is the difference between "ketika" and "saat"? In the multiple choice question I've got "Dompet saya hilang saat saya berangkat sekolah"...….

Also, shouldn't it be "berangkat ke sekolah"?

Maybe in Indonesian the difference is not significant(?), but in English it is. - Dompet saya hilang ketika saya berangkat sekolah: My wallet was lost when I left school. - Dompet saya hilang ketika saya berangkat ke sekolah: My wallet was lost when I left for school.