I've shouted at school receptionist before, after she told me she could not interrupt a meeting the teachers were having, I asked her of the meeting was important and she said it was the monthly staff meeting.
Before this, I stated that I has to phone an ambulance for my son, as he left school clearly having an allergic reaction, by the time we got him home and put him on his nubuliser, we realised he needed emergency treatment...
Ds was having great difficulty breathing and had hives and swelling on his face arms and torso and legs....
but apparently she could not interrupt a meeting... 
she got it both barrows.... turned out hay was being delivered when ds was leaving his class room, ds is allergic to grass!
I got a written apology for that one.... and an almighty red faced head.
Its not something I normally do, but sometime needs must.