DENVER – A bearded drifter walks into a Colorado school and fatally shoots a student before taking his own life. Wisconsin authorities charge three boys with plotting a bomb attack on their high school and, two weeks later, a student in a rural school allegedly shoots his principal. A gunman bursts into a Vermont elementary school looking for his ex-girlfriend and guns down a teacher.

All of this in the past month alone.

Since the 1999 Columbine massacre that left 15 people dead, there has been a determined effort among administrators, principals and teachers to improve school safety. Law enforcement officers across the nation and around the world have added training specifically intended to address school violence.

But experts say there is simply no way to guarantee that a stranger or student won’t be able to injure or kill on school grounds.

“There’s no perfect security, from the White House to the schoolhouse,” said Kenneth Trump, president of the National School Safety and Security Services consulting firm in Cleveland.

Since Columbine, school officials have gotten better at preventing student violence, he said, but authorities can’t prepare for every problem.

“When you factor in unpredictable outsiders, when you have a roaming monster walking into the schools, we have to be realistic,” Trump said. “There are some incidents you’re not going to be able to prevent.”

Trump’s firm counts 17 nonfatal school shootings so far this school year, beginning Aug. 1. There were 85 the previous school year and 52 in the 2004-2005 school year.

Since Columbine in 1999, the number of fatal school shootings in a school year has ranged from three (2002-03) to 24 (2004-05), according to National School Safety and Security Services. The firm does not track cases before Columbine.

Park County Sheriff Fred Wegener was among the law enforcement officials who eagerly applied for federal aid to beef up security at Platte Canyon High School in Bailey, the site of last week’s attack in which a man held six girls hostage before killing one and himself.

A deputy was assigned to be the school’s resource officer – essentially, its security guard. But that guard was called away on sheriff’s business last Wednesday and gunman Duane Morrison walked inside with two handguns. He reportedly sat in the school parking lot and wandered the hallways for as long as 35 minutes before the siege began.

Despite the death of 16-year-old Emily Keyes, things could have been worse, authorities said.

“Basically, the tragedy of Columbine taught law enforcement and educators how to avoid future tragedies,” Gov. Bill Owens said. “In a couple of significant ways, the tragedy of Columbine may have helped prevent an even worse tragedy (here).”

He said educators had been instructed in August on what to do. The school was also designed using concepts learned from the Columbine attacks, which helped authorities keep the gunman in one room.

Ever since Columbine, school officials have been taught to write emergency response plans and practice them, to lock down schools and evacuate when it appears safe. That seemed to work well in Bailey as hundreds of students were whisked to safety.