MILWAUKEE (CBS 58) -- Riverside University High School students' first day back to school will be delayed as Milwaukee Public Schools says the high school will be closed Tuesday, Sept. 2. 

They say cleanup efforts are ongoing after a chemical spill that happened last week. 

A message sent to families said, in part, "We value your health and safety and are happy to report that our building passed its air tests after the spill. However, a few steps remain in the cleanup process and a few classrooms will be closed."

MPS says they're still working to find out what caused the spill, but one factor may have been old science equipment containing mercury. 

Staff is expected to report to work Tuesday, and students are expected to be welcomed back for the first day of the school year Wednesday, Sept. 3.