PORT HURON, Mich. (AP) _ City schools closed last week after the arrest of four students who allegedly planned a massacre similar to Colorado's Columbine High School slayings will reopen Monday.

Police will be posted inside Holland Woods Middle School, where a bomb was found Thursday after school officials and parents gathered to discuss the alleged plot by the boys. Police have said the bomb is unrelated to the alleged plot.

All of the district's 21 campuses were closed Friday, then searched over the weekend and found to be safe, superintendent Bill Kimball said Sunday. Security will also be tightened district-wide on Monday, and teachers will discuss school safety with students.

``We're taking every measure possible to be sure our kids are safe,'' Kimball said.

Four Holland Woods students ages 12 to 14 remain jailed after being charged on Thursday with conspiracy to commit murder at the 560-student school.

The two 14-year-olds, Justin Schnepp and Jedaiah Zinzo, have been charged as adults and are being held on $100,000 bond each. The younger students are to appear Tuesday in St. Clair County Juvenile Court.

All were arrested last week after a 14-year-old girl told police she overheard some of them talking about an elaborate school killing spree.The girl told police the boys said their plan was to go on a shooting spree in a gym assembly and detonate a bomb afterward, acts aimed at killing the school's ``preps,'' the popular clique of students.

Again Sunday, police and school district officials declined to comment on evidence in the case. St. Clair County prosecutors have also declined to comment about evidence.

Officials have said the teens planned to top the death toll of the April 20 massacre at Columbine High School in Littleton, Colo., where teenage gunmen Eric Harris and Dylan Klebold killed 13 before committing suicide.