As an ex-teacher, I admit that I tend to sympathise with the school/teacher in a lot of threads but this is completely unacceptable and I'm horrified by the school's response.
11 is still very young and they've only been in school for a while. Children often do listen to teachers unquestioningly and assume they know what they're doing.
I've always had a terrible sense of direction to the point I get lost in buildings I've just entered, don't know my way around outside of a very small area that I live and generally can't remember which turn I've taken as soon as I've taken it. I only found out, after a late (adult) diagnosis, that it is due to dyspraxia.
I still remember the fear when teachers would keep us late, telling us that they were legally allowed to keep us for ten minutes without informing our parents and when we pointed out that the school buses leave at that time, they would just laugh as if they didn't care.
I was constantly filled with dread about missing the school bus because the alternative was to get a bus, walk, then another bus and no matter how many practice runs I took with my mum, I just couldn't remember the way.
I honestly don't think teachers should be allowed to keep children for any amount of time after school unless it's a formal detention and at least 24 hours notice has been given to parents.
I feel like a lot of pps are giving an 11 year old a hard time about not speaking up. Why did the teacher assume these children were all getting the same bus? If the rule is that phones need handing in, and presumably all the children own a phone, why didn't they take them to collect their phones? Honestly, it seems like a massive safeguarding issue and the attitude of "well he should have said anything, not our problem" would absolutely fill me with rage.
I would be taking the formal complaint avenue and expecting a policy change so that nothing like this happens again.
I would advise getting your DS to memorize your phone number, getting some kind of tracking tag for his blazer/bag and practising speaking assertively when needed. Sometimes children are worried about getting in trouble if they argue/disagree with teachers, so assure him that you will back him up 100 percent to the school if he ever feels like he's in a worrying situation again (even if he turns out to be wrong). He made a sensible choice in sticking to where he was, knowing you would come so praise him for that.