Highland Park High School was evacuated Tuesday morning after another threatening note was found on campus.
Classes were canceled for the remainder of the day, the school said.
Officials have not disclosed the contents of the note or who was targeted.
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The school district said in a statement later Tuesday that classes would resume Wednesday with normal school hours.
Students are asked to enter the building through either the front doors on Emerson Avenue or the student entrance on Westchester Drive. All other entrances would be locked, the district said.
In January, three threatening notes were found on consecutive days in a boys bathroom. Classes were canceled the first two days, and sweeps of the building turned up nothing out of the ordinary. On the third day, classes remained on schedule.
The district said in its statement that it did not know if Tuesday's hoax was related to the January incidents.
Police said in January they would pursue felony charges against the person responsible for those threats.