More than 30 seniors from Copiague High School will be barred from next week’s graduation, according to parents who said Friday their teens are being punished for acts of vandalism committed by a relative few.

Copiague school officials said, however, that some students’ actions went far beyond mere pranks and included defacing school property with eggs and shaving cream, ransacking an assistant principal’s office and smashing an expensive projector used in instruction.

School authorities said they would not bring criminal charges, but regarded the incident as very serious and potentially hazardous.

“The behaviors exhibited by the individuals involved in this incident are unacceptable and inexcusable,” said district Superintendent Kathleen Bannon, who added that such actions “do not represent the fine character and wonderful accomplishments” of most seniors.

The break-in at Walter G. O’Connell Copiague High School occurred at 2 a.m. on June 9, setting off alarms and alerting Suffolk County police, the district reported. Security-camera tapes recorded the incident, according to the report, which said some teens entered through a second-story window.

A Suffolk police spokeswoman said the district informed her department it would handle the matter internally.

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Copiague officials declined to say how many students were disciplined or what punishments were imposed.

Parents and students told Newsday that about 70 seniors were involved, and that most showed up at the school for what they assumed would be a traditional year-end prank — nothing more serious than scattering balloons and paper streamers. However, six students wearing masks engaged in serious vandalism, those interviewed said.

Parent estimates of the number of seniors barred from graduation ranged from 33 to 40. Most of the six parents and students interviewed said they had heard that not all vandals were identified, and that some would likely participate in the graduation.

Copiague High enrolls about 1,600 students, including 388 seniors.

Several mothers vowed to appear at a 7:30 p.m. school board meeting Monday at the middle school to plead for their teenagers’ right to attend graduation. Graduation is scheduled for 7 p.m. Thursday on the high school grounds.

“My daughter came to play a prank with a balloon,” said one of the parents, Jolanta Bazyte. “I don’t defend it. But I do think her 12 years of showing up, trying her best, gives her the right to walk at graduation.”

Bazyte described her 17-year-old as a National Honor Society member with a 90-plus grade average.