I don't think it is "just" a form of control to get what he wants because a child who is generally ok is not going to do this. I do think this age for adopted children is quite pivotal, incidentally.
I agree with a pp that the natural consequence is not him not going to school, because that puts far too much responsibility and pressure on the child - I think that the natural consequence is that you find a different way of getting him to school while also working out what the problem is - if he really likes the bus then he may go back to getting the bus, and not using it as a way of trying to get what he wants in other respects. Though you would still need to work out the behaviour.
The other key thing is working out what the behaviour is communicating - to work out why he is doing it and deal with the root cause.
Do you generally have a good connection with him, and after arguments is it easy for you both to say sorry and reconnect?
When you have applied PACE - you being playfully curious, accepting, empathetic, about what is really going on - any clues from him? If not, then are you able to think about it yourself, think about when it started, anything else going on at the time, manage to second guess until you find the answer?
You say it isn't anything to with school or the bus but it could be - it could be that he is getting bullied, or feels uncomfortable, or is trying to get a reaction from you (if so, why?) or because he wants you to take him to school, he may be being abused and this is a way of trying to communicate it with you, or any other reason related to the school or the bus.
Or it could be that he is very unhappy about something not at all related to school or the bus.
So difficult!
Is he comparing himself more to his contemporaries in terms of families and family life? Age 12 can be a pivotal age in this respect. There is a lot of chat on buses about life outside school and it might be that which is upsetting him.
To take the positives, hopefully it is a good thing that he is being this assertive with you, if it means that he trusts you, to help him get to school and work out whatever the problem is.
Unless he is being difficult because he no longer cares about the impact in which case that would be more worrying, I think. And you would then have a bit of work to do with connection.
What does his therapist say? And is the therapy the right therapy for him at the moment?
It could be that he feels out of control because of a lack of boundaries, or that he wants more control in his life, appropriate for his age, if the "therapeutic parenting" methods of limiting choices etc are too restrictive, too over managed.