GUILDERLAND — According to Brian Forte, what happened last week on the Guilderland High School football field is not a senior prank.

“It’s criminal mischief,” said Forte, a Guilderland Police officer who has been stationed at the school since 1999.

The sod was dug out of a 20-by-20-foot patch in the end zone; the numerals “09” were dug into the earth and filled with cement, said Forte.

Have the perpetrators been caught? “Not yet,” said Forte.

In the decade he’s been a resource officer at the school, Forte said, the last week of classes have always brought pranks, some of them rising to the level of crimes.

Last year, painting on the front of the school “absolutely” rose to the level of crime, Forte said, costing thousands of dollars to clean. When the damage tops $250, the criminal-mischief misdemeanor charge becomes a felony, said Forte.

In 2006, the high school library was emptied of many of its books and “The Class of 2006” was written on walls and a ceiling. The vandals, who broke into the school overnight, were never caught. Senior volunteers later put the books back in the library but staff had to spend hours getting them in the right places.

“They become very tight-lipped,” Forte said of interviewing seniors to find the vandals.

For years, Guilderland High School seniors climbed the water tower near the school — built in 1941 as part of the old Army depot — to paint colorful class slogans.

“Every year for probably as long as the tower was there, they’d do that,” said Forte.

Some of the slogans in recent years were: “Life’s a trip. Enjoy the ride” and “Life’s short. Party naked” and “Life’s a blow-pop. Suck it.”

The tower was dismantled and removed in November 2006.

Forte cautioned students against destructive acts. “Anything that is disruptive or causes damages is a burden to the community — the school and the community at large,” he said.

He went on, “I don’t know where this rite of passage, and the idea that people are entitled to do this, comes from. How is it a prank if it violates the law and causes damage?”

Something Forte said he would consider a prank as opposed to a crime took place on Friday, the last day of school. “They took soap and wrote on all the windows in the front of the building ‘Seniors 09,’” he said of the seniors.

“Before they went to their class picnic, we had them clean the building off,” said Forte. The “vast majority” of seniors volunteered to help with the cleanup, he said, concluding, “No damage was done.”