STOUGHTON, Wis. -- Stoughton High School parents are upset the district did not inform them of the hazard until hours after it happened.

Superintendent Myron Palomba says no one knew about the spill that happened at 9 a.m., but administration responded immediately when they were informed about a second incident at 1:15 p.m.

Over the weekend, the school was cleaned and other forms of mercury that were discovered were removed from classrooms.

"We're going to take a look at how we can improve in the future if we get into situation in the future better communication to parents," Palomba said.

School officials told students to call their parents right away and found out some did not.

Urine tests results on 40 students are expected by the end of this week. Mercury is poisonous to humans and can affect nerve and brain function, including memory loss and hearing problems. Exposure to pregnant women can cause birth deformities. ( Read More About Health Effects )