PORT HURON, Mich. (AP) _ The week's first lesson for every Port Huron Public School student will be the same: how to feel safe following the discovery of a bomb near one school and four classmates' arrests in an alleged mass-murder conspiracy.

``Every teacher will spend a considerable amount of time ... talking to their students about this incident, and we intend to do this in every school in every grade throughout the school district,'' Superintendent Bill Kimball said Sunday.

All 21 schools reopened today with extra security in this city about 60 miles northeast of Detroit. Extra guards posted outside Holland Woods today searched students' backpacks before allowing them into the building.

Schools were closed after the bomb was found Thursday by a custodian near Holland Woods Middle School. Police say it is unrelated to the arrests of four students on Wednesday and Thursday who were charged with conspiracy to commit murder.

Still, some remained worried.

Whitney Fontenot, 12, a sixth-grader at Holland Woods and her 10-year-old sister, Heather, cried at Sunday's news conference. ``I feel uncomfortable going to that school,'' Whitney said.

``I think that they should just end the school year and have that extra three weeks to figure out what they're going to do to safeguard our kids for next year,'' said Heidy Martindale, the mother of a 12-year-old girl at Holland Woods.

Assistant Superintendent Tom Miller said social workers will meet with students. ``Our goal is to try to attempt to provide them with anything that they need so that we can get them back into a normal day and a normal conclusion of our school year,'' he said.

Last week, Justin Schnepp and Jedaiah Zinzo, both 14, were arrested, charged as adults with conspiracy to commit murder, and each was ordered held on $100,000 bond.

Two other suspects were charged as juveniles with the same crime. One boy is 13; police said the other boy is 12, while a prosecutor said he is 13. They are to appear Tuesday for a hearing in juvenile court, police Capt. Brian Moeller said.

Moeller would provide no details except to say the boys were not armed when arrested at their homes.

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

Elsewhere:

_ In the Dallas suburb of Allen, Texas, school officials who last week said they were suspending the final two weeks of classes after repeated bomb threats said students would return later this week for staggered classes. Officials said Sunday that they only wanted to confuse those calling in the threats and weren't canceling the rest of the year outright.

``There has been a lot of miscommunication,'' complained a parent, Scott Gleason. ``If they weren't really closing down, why the heck did they let this get publicized for two solid days?''

_ In West Palm Beach, Fla., Saturday's prom at Palm Beach Lakes High School ended early when a nail-studded pipe bomb was found hidden inside an indoor tree planter by the hotel's ballroom entrance.