For the amount of red liquid that had been sprinkled over the party goers,it wouldn't be necessary to taste it to know whether it was or wasn't blood. Even a very small amount of blood does have an intense metallic smell similar to rust or copper, which comes from a high content of iron reacting with oxygen in the air.
It has been claimed that activating the fire alarm pull would not set off the sprinklers, it would merely trigger the fire alarm to sound and also if equipped alert emergency services.
Sprinklers are activated by the heat of the fire and only the ones that the heat triggers are set off, not all of them at once.
However, that is not the only system and it would do nothing to protect the school full of children or the irreplaceable, largely flammable antiques. So in a place where teenagers are performing rituals with lots of candles and it's entirely possible to have "Firestarters" (pyrokinetics) as students, it would make sense to have a system (that exists in real life too) where pulling the alarm activates the sprinklers in the entire room.
At about 1:48, when Thing is photocopying a copy of the autopsy report, the first page when blown up says "County of Los Angeles" and "Department of Coroner." The morgue is identified on the outside as "Jericho County" and "County - Medical Examiner".
At the end of the school dance, fake blood is pouring from the sprinkler system. Wednesday tastes the liquid and states that it is no "real pigs blood" but colored water, previously she also explains she is allergic to color or pigments "i break out into hives every and then the flesh peels off my bones" which is inconsistent with her not responding to the pigments on her skin.