Depends on if it’s serious or not. If it’s minor then we usually send an email or a note home. Something serious gets a phone call right away.

I busted my head open one day at school (bad enough that i needed 6 staples) and they never called my mom. They waited until she came to pick me up, and then told her that there "was an accident, but it wasnt that bad." My mom was livid

I’m sorry, the school nurse or someone should call you? Is that the question?
Most times they don’t call immediately unless it’s severe. Kids I believe go to the nurse and then go from there.