Nearly 2,000 Columbine High School students returned to class today following a festive outdoor rally featuring cheerleaders, pep songs, a ribbon-cutting--and no remembrance of the victims of the mass shooting in April that claimed the lives of 15 people.

"I have waited for months to say this, and I say this with great pride," principal Frank DeAngelis said. "Columbine, we are back!"

Then staff and alumni cut a ribbon at the school's front entrance and raised to full staff a flag that had been lowered since spring to honor the dead of the worst school shooting in American history.

However, the failure of school officials to include in the "Take Back the School" rally any tribute to--or mention of--the 12 students and one teacher killed by Eric Harris and Dylan Klebold before they took their own lives angered parents of some of the victims, who accused administrators of insensitivity and a failure to bolster school security in the wake of the killings.

"I don't expect them to wallow in it, but this is a slap in the face," said Rich Petrone, whose stepson Dan Rohrbough died in the assault. "I expected them to at least take a moment of silence. The school district, they are still in denial. To me it was just rah-rah, let's forget about the kids who died."

Rohrbough's father, Brian, said: "You can't move forward without acknowledging what happened. I'm very disappointed in the school board and with Frank DeAngelis." Accusing the school district of making only cosmetic improvements to security, Rohrbough said, "My only child was murdered here. I am very fearful that what happened here will happen again."

School officials here have spent many weeks studying ways to make Jefferson County schools safer and tentatively have approved plans to add armed guards, install more video cameras and limit access to school buildings--changes that already have been made at Columbine to allay community fears. Three armed officers patrolled the halls at Columbine this morning, and mental health workers and a "safe room" were also in place in case any students were traumatized by returning to the school that has been closed since the killings.

"We were very pleased with the start we had to the school day," said area school administrator Barb Monseu, adding that there was no intent to slight the families of victims by not recognizing those who had died. "Today our focus was on getting the students back."

Gathering in the parking lot where Klebold and Harris began their assault on the school, Columbine's students showed little emotional reaction to returning to the building where so many of their classmates had died--other than the usual exuberance that accompanies the first day of school. Mingling under brilliant blue skies with the foothills of the Rockies gleaming in the distance, most of the students wore T-shirts emblazoned on the front with "We Are . . ." and on the back with "Columbine." They eagerly joined in that traditional school chant when social studies teacher Ivory Moore led the cheer from a podium in the parking lot.

In what appeared a tacit acknowledgment that school officials had not been sensitive enough to the kind of social ostracism felt by Harris, Klebold and other students outside the mainstream of the suburban Denver school, DeAngelis announced today that Columbine would adopt a "zero tolerance" policy for "cruelty, harassment, excessive teasing, discrimination, violence and intimidation."

Calling on students to show respect for others and to communicate more openly, DeAngelis said: "At Columbine High School we can no longer state we were only kidding when we made inappropriate comments or exhibited inappropriate behavior. Derogatory comments, verbal or written, made about others will not be tolerated. Racial, religious and sexual slurs or jokes will not be condoned. . . . And I hope the compassion and the unity you had at the end of last year will continue and be built upon to give us strength to move forward."

School officials went to extraordinary lengths to impart an upbeat mood to today's first day of classes, sharply restricting access to the general public and media, with all but a handful kept several hundred yards away from the rally. Many of the school's students said in recent days they were eagerly awaiting a return to normalcy after living in the glare of publicity for so many weeks.

By all accounts, this was about as normal a day as could be expected at a school where less than four months ago a slaughter took place.

"It was not exactly like a regular day at school," said sophomore Peter Tognetti, 15. "But it felt a lot more normal than it did in June when we went back to get our stuff and it was more like a crime scene."

Tognetti and other students said the subject of the April 20 shootings came up only rarely today and usually in the context of discussing DeAngelis's admonition to be more respectful and open with each other. Except for members of her choir class who "have a tight bond" after being pinned down together in the choir room as the two gunmen rampaged through the school, said senior Kathryn Ulibarri, "no one really talked about it much."

English teacher Paula Reed admitted that "some of us started back with some trepidation," but "we were pleasantly surprised when what we saw were a lot of smiling faces."

Two weeks ago, said Reed, the nightmares she regularly had in the spring returned, but vanished again when she came to the school earlier this week to prepare for classes. "To walk through halls of kids laughing and having school, I appreciate that more than ever," she said.

CAPTION: Parents form a human chain to shield returning Columbine High School students from the media on the first day of classes since April's shootings. "Columbine, we are back!" said the principal at a rally that the stepfather of one shooting victim called "a slap in the face."