The Ormiston Rivers Academy in Burnham-on-Crouch warned parents yesterday (June 2) about the revelation.
As a result, they are reported to have told parents to get their children tested for the virus urgently.
A number of Year 11 students at the school are also understood to be self-isolating with the school also in consolation with Public Heath England.
Following initial reports, the school has now confirmed the news.
They say that, as pupils remain in strict bubbles during the day, the spread of the virus likely occurred as Year 11 students met up outdoors after their final day before half-term.
Despite the Covid fears, the school says it is safe for them to remain open for other students and will be safe to reopen following the break on Monday, June 7.