i'll declare now I work in a school doing the first aid.
What a distressing incident for you and your child - I hope you are all ok. The absolute worst bit of my job is when a child gets a head injury. When they are safely gone with their parents I always cry. So goodness knows how a parent feels.
I wouldn't expect a child to be left alone - I would expect someone to be applying pressure to the wound and keeping a close eye on them.
I understand really understand why they said 'may' as when I phone, it is always with the child sat right next to me and I don't want to freak the child out. I also don't want the parent to drive panicked.
The ambulances triage and a conscious patient, talking and responding, where the bleeding is under control is quite low down the list - generally well over an hour from experience - but without seeing it and how disorientated he was I couldn't tell whether I would have called one.
It is concerning that the teacher didn't come and pick up on his altered state / confusion. (a receptionist probably wouldn't know your child well enough to realise they were any different) but we always get someone that knows them to come and have a chat for a big knock to the head.
Surrey Health produce a great head injury leaflet that has a green, amber, red scale - google it - it might help you schools decision making.
I think it sounds like the school could review what they do especially the leaving alone.