The person who sent the note was apparently upset about how police handled the situation.
Parents were notified by e-mail about the threat, and as a precaution, there was extra security at the school Friday.
"There's a lot to be concerned about, but I feel better about what was said and what's being done about it," said parent Janine Marzette.
"I want to make sure they're getting the information directly from me, and they're getting the truth about what's going on at the school. They feel confident about that now," Principal John Price said.
Officials say Chicago Public Schools and Chicago police are working together to make sure the students are safe.