MM,
How horrible for you. You trusted that your son was in sight and/or sound of the teachers, which is a legal requirement. I'm a preschool teacher and would be upset if someone hadn't even checked the bathroom as well, knowing the lights were out, even if it was connected to the classroom.
What concerns me more, however, was that it took them 40 minutes to call you. Forty minutes when his head was bleeding to the point it needed to be stapled? Personally, I'd be furious about *that* end of it. Yes, I'd call to report this right away. In what world is waiting that long to call a parent okay? Were you in a meeting? Whether or not he was climbing up where he shouldn't have been should affect how long it took them to get a hold of you.
I hope your son feels better soon. Were something like this to have happened to a child I cared for, I'd have called the parent as soon as I'd ascertained there was an injury and applied first aid, but immediately. With three teachers, one of them should have called you right away.
Are you looking for a new daycare yet? If I didn't trust the provider, that would be my first move before I did anything else.
(FYI: The insurance policy I carried on my preschool basically stated that if I even *thought* something had the slightest potential to be harmful and I neglected correcting this, I was liable. I'm pretty sure the daycare's insurance is similar, because insurance companies don't want to be paying out on the kind of insurance policies we carry. It's too expensive for them. They'll likely fight it, and you may have to lawyer up to get a settlement.)