My son's teacher tried to stop my then 16-year-old from collecting him from his classroom one day a week, walking him through the playground and out of the gates... where I was always stood waiting with the dog. Yet were perfectly content to allow the 12- and 13-year-old siblings of my then 8-year-old's friends literally collecting them from school and walking them home, without any parents present. There weren't any safe-guarding issues with my solution to the one night of the week where my son stayed late, and the dog needed to be walked (but, obviously, couldn't be on school premises) - but they tried to make some, believe me. Until I pointed out that I knew for a fact that they were allowing much younger teenagers/adolescents to assume full responsibility for 8-year-olds on the actual walk home (along a main road, most of it without any pavement - it's a rural village, but we are used as a cut-through by a lot of traffic, for who knows why!) - so why not deal with that, then the "issue", the "safe-guarding concerns" of literally collecting from the classroom after the club was finished, and walking them through the gate to the parent (and no; I couldn't not walk the dog, or leave him with my daughter, as he required a walk at that time of day, and literally would have been dragging my daughter after me if I'd tried leaving him with her...). It's bizarre, because they were perfectly happy with my dropping my son at the gates and trusting him to walk himself to the classroom every morning. Some teachers/schools are a bit OTT and inconsistent about it, to boot.
Do the school know why you and your children had to move away, @Livelove0x ? Might this have a part to play in why they're concerned now (although how on earth did they even find out?!)? I would think that as long as your children are both in school, on time, and are otherwise clean and healthy, they can't dictate to you about a short bus ride for these things to happen. As a single parent (and I'm one, too, so I get it) of course you have to work, otherwise your children suffer as a result of there not being adequate food etc., for them. Point out that you are in constant contact during the bus ride, and that you - as the parent - have full confidence in your older child's sense of responsibility. That you are encouraging independence and resilience in your children (which you are), because you don't expect them to wait for everything to be done for them, because "they're just kids". Your youngest's nursery is nearer to your current home, but you haven't changed your other children's schools because you wanted them to have a sense of continuity given the current set of circumstances that through no fault of your own, or theirs, you have all found yourselves having to find a way through.
Christ, though, being up in arms about two kids on a bus without adult supervision...! 
How very much a sign of society today, that is. 
I reckon that it's one of the other parents on that bus ride who alerted the school to what you're doing. Someone who probably thinks that they're helping, but because they've stuck their nose in where they actually have no need to, they're just adding to your problems right now. 