A student who police say was armed with a knife has been Tasered and arrested at a high school in Sydney's north.

Officers were called to North Sydney Boys High following an "altercation" between two students in a classroom about 12.30pm today.

Police said one of the students left the classroom armed with a knife.

The school was placed into lockdown for an hour as police searched for the 15-year-old student, before he was found by officers, then Tasered and arrested.

The boy was taken to hospital, and the lockdown lifted about 1.30pm.

The NSW Department of Education said the lockdown had been used "as a precaution".

"All students and staff are safe, and students resumed classes after 1.30pm," a spokesperson said.

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