I really hate the "SS will get you if something happens" line - it gets trotted out in every situation in which an under 18 might be out of parental eyesight, from the sublime to the ridiculous, whether the topic is is "would you leave your 2 year old to play alone next to the M1 while you went on a 3 week holiday" or "would you let your 15 year old walk home from school alone?"
If "something" happened the issue would be that "something" had happened (presumably something unfortunate) - whether or not SS would get involved should be the last thing on anyone's mind - why are they trotted out as the boggyman to scare naughty parents into behaving in some particular paranoid/over protective/ smothering/ cautious/ sensible (depending on the thread) way? Surely the issue is whether the child will be likely to be safe and gain anything from the experience?
That said the answer is really it depends - bus door to door, stopping right outside the school, sensible child happy confident and eager to take the responsibility, route practised multiple times in advance with parent, perhaps one of those small local buses not a massive city double decker, route known to be quiet and not packed - seat guaranteed - in all honesty why not? In other circumstances (very busy route, stop not immediately outside school, no preparation, child worried) - of course not.
Mine take the school bus from age 6 - walk to the stop on their own, turn up home having caught the school bus and walked from the stop. As does every other child, because we live in a country where that is how it works. From age 10711 when they change schools they take a public bus technically, though in fact it is full of school kids and rarely anyone else, and there is only one bus each way per day...