Would they? Around here only kids with disabilities are bussed, and the buses go door to door and the aides walk the kids to their door -- I'm not sure at what age they'll leave if noone's home, but the kid has a key. In the neighboring school district I think it's 8.
However I also have a goddaughter who got off at the wrong stop on the first day of Kindergarten, and noone stopped her. As soon as she recognized where she was she told her bus driver "I see my house" and he let her off -- 3or so blocks before the stop for her actual house where her Grandma was waiting for her. Luckily she just walked home, but crossed at least one super busy street in the process. So my vote for "would they" is 5.
To those people who say that there's no way to check -- my son goes to camp on a school bus, that picks up 15 or so kids at each site. For the little one's (4 and 5ish) you walk up to the door, say your chld's name and they let them off. For the older kids, they just tell the driver "I see my mom" and point. If noone's there for a specific child (most of us have arrangements with other parents -- if for some reason I don't make it another mom would "accept" my kid and vice versa) then they call on the cell, and if there's no response they take them to the next stop. I know they wouldn't let my 9 year off unsupervised unless I specifically told them too, and I'm pretty sure that's true up until 14.
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