Is a nosebleed bad?

Apparently my 6 year old has had two 'bad' nosebleeds at school in the past 3 days. His teacher keeps expressing that they're true nosebleeds not just tricklings of blood. I'm a bit nervous. He eats good and takes Attentive Child for concentration but nothing else. I did get a doctors appointment but has anyone else experienced this? What could it be?