At around 2.30pm on Wednesday (July 7), officers were made aware of a pupil in possession of a knife and behaving in a threatening manner at the secondary school in South Farm Road.
A Sussex Police spokesman said: “The matter, which is being treated as an isolated incident, was dealt with at the time by the school and no one was harmed.
“Officers are now carrying out a number of subsequent enquiries in liaison with the school.”
Pan Panayiotou, head teacher at Worthing High School, confirmed it was an isolated incident and involved a student using a knife obtained from a food technology lesson, behaving in a threatening manner.
He added: “The school went into partial lockdown, whilst the issue was dealt with.
“This was an isolated incident and the rest of the school and the students were protected and safe, while the issue was being resolved.
“No one was injured. The school has provided support for all those involved and the student concerned.”