Well, the day your child is sent to the emergency room and everyone but you is notified, is the day you can say, I support the school.
It's the schools OBLIGATION to notify the parents anytime there is an emergency.
My son was sent to the emergency room from school a few weeks ago. The school was jumping thru hoops to try and get a hold of me. I kept my son home from school that day. We were both not feeling well, but he was watching TV and I told him I wanted to lie down for a bit. He started feeling better, and went to school without telling me anything, OR bringing me my cell phone (no landlines anymore). He later said he wanted to let me get some sleep. Well, the school tried and tried to get a hold of me, but couldn't. They even had students calling me (friends of my son's) to let me know they were sending him to the hospital. They finally ended up calling my husband at work because notifying a parent is priority #2 (#1 is getting help on the way like an ambulance).
There is no excuse or justification that the OP was not notified. None whatsoever!!
They did make contact. The almost adult daughter informed the school that her parents were out of town. The school called the sister, proper procedure.
The sister called the OP. Again, emergency contacts working as they should be working. That is why you have emergency contacts listed, so that they can be relied upon to contact you if the school cannot. Since the school was specifically told the parents were out of town, the school did what they were supposed to do and contacted the sister.
Per OP's first post, she called the school back within 30 - 40 minutes. Again, communication working properly.
During that half hour between the sister being called and the OP getting in touch with the school, something happened to make the school decide to call 911.
It is not unreasonable to believe that 30 minutes is just enough time to call the ambulance, wait for the ambulance, stay with the student until transported.
If the school hadn't called by the end of the day, then I could support the OP calling the nurse to first of all thank her for helping her daughter, but also to inform the nurse that they needed to make sure the updated forms were in her daughter's file.
Again though, with the title of the thread;
DD was sent to the hospital from school via ambulance without our knowledge (OP annoyed that an ambulance was called at all without their knowledge)
and with her first post where she states that the Nurse should have called her before she called the ambulance, it is clear that the OP is not upset about not being called after the fact, but annoyed that the school didn't call her before calling an ambulance.
It was only after a page of answers saying that the school did the right thing by calling 911, did the comments of being angered about not being called after the fact pop up.
OP, glad your daughter is ok

. Be glad that the school erred on the side of caution and did call the ambulance. 20/20 hindsight is a wonderful thing and now you can look back and say "she was only out for 30 seconds." Be thankful that it was a mundane trip to the ER rather than a more serious emergency.