St Leonards Academy said all students, including those in year 11, must stay home for two weeks.
The school had previously sent home years 9 and 10, on Sunday, before extending this to include years 7 and 8 on Monday.
However, on Wednesday (December 2), the school said another case was confirmed which resulted in the school being closed to year 11 students as well.
The school will remain open for children of critical workers and vulnerable students.
The closure is with immediate effect. Years 7, 8, 9 and 10 will return on Monday, December 14, while year 11 students will return on Tuesday, December 15.
The University of Brighton Academies Trust, which runs the academy, said: “It is with regret that following another confirmed case within our academy community. We have taken the difficult decision to close the Academy to all year groups with immediate effect.
“We will remain open to children of critical workers and vulnerable children.
“The decision to close is due to further staff members now being required to self-isolate. Which means we are unable to maintain the appropriate level of staffing needed to operate the Academy safely at full capacity.
“We will now move our learning online to support our students during this closure and look forward to welcoming back our Year 7, 8, 9 and 10 students on Monday 14th December and Year 11 students on Tuesday 15th December.
“We want to take this opportunity to thank all our families for their continued support and understanding during these challenging times.”