Brother Gary Cregan, principal of St. Anthony's School in South Huntington, said there was no history of conflict before the racist incidents, and school officials did not know what provoked them.
Two senior boys were expelled for bringing a Confederate battle flag to a European handball game April 9 and displaying it. Teachers confiscated the flag, but a photo showing the flag in a school hallway with students around it was posted online.
Cregan said two sophomore girls also posted a photo on social media that showed one of them in blackface. The photo was accompanied by a racist text.
The boys were originally suspended for 10 days, but Cregan said he decided more severe punishment was needed. He said the pair brought "disrepute" to the school.
The blackface photo was posted last week.
"I had hoped, perhaps naively, that we would be able at a given point to reasonably sit down, but the emotions are still very high," Cregan told Newsday. "You can feel the mood in the building. The mood is not a pleasant mood. It's a tense mood.
Cregan said about 75 percent of the school's 2,450 students are white. The school has 138 black students, 215 students of Asian descent and 200 Hispanics students, with a few students of American Indian or Polynesian descent.
The school, on the north shore of Long Island, was founded by the Franciscan Brothers in 1933.