Pupils at a private school tried to force-feed a piece of pizza containing ham to a Jewish student.
It happened at Clifton College, Bristol, the school confirmed, while the student was eating with his "close-knit group of friends".
The Bristol Post reported a pupil was given a short suspension afterwards.
The college said the victim felt it was not an anti-Semitic act but that it considered it to be "completely unacceptable".
The incident last December fell "far short of the high standards we expect from our pupils", a school statement said.
"Anti-Semitism in any guise is abhorrent, pernicious and is absolutely not tolerated at Clifton College, which enjoys a deep and longstanding connection with the Jewish community.
"We are proud that there have been Jewish students at Clifton College since 1879. They are an integral and valued part of the Clifton College community."
Stephen Silverman, a trustee and director of the Campaign Against Antisemitism, said this would have been an "incredibly traumatic and abusive and offensive" act for an observant Jewish person who closely followed the dietary laws.
But, he added, only the victim himself was in a position to judge if it was anti-Semitically motivated.
"One can only speculate as to why the children concerned thought it was funny to do this and whether they would have done it to anybody else," he said.
"One would hope if the school had decided there had been anti-Semitic intent, they would have imposed a far harsher penalty.
"I suspect it was mitigated by the position of the victim."
The school said a "thorough investigation" had taken place and "proportionate and appropriate sanctions" put in place.
"We take every opportunity to educate our pupils about the importance of kindness, tolerance and the potentially devastating effects of prejudice," it added.
Clifton College's website describes it as a top independent boarding school, with 1,250 pupils, including 720 in the Upper School.
Termly fees range from £561 for the nursery, to £13,135 for some sixth-form boarders.