Parents have been told students will be tested at school next week, and advised to get home kits for children already in isolation.
Mobile testing units will be based at the school's Shooting Field and Rock Road campuses.
The school's Shooting Field campus in Steyning closed on Monday (June 14) after two Covid cases were confirmed - one student and one member of staff - and another seven people tested positive using the Lateral Flow Tests.
Some students at the Rock Road campus in Storrington were later told to isolate as well after a confirmed case there.
A letter sent to parents by the school said: "As you may be aware, we have been informed by Public Health England (PHE) and West Sussex County Council (WSCC) that, following a rise in case numbers in the Storrington and Steyning areas, a number of coronavirus (COVID-19) cases have been identified at the School. Some of these confirmed cases have subsequently been identified as the variant of concern first identified in India.
"As a proactive and precautionary measure to help limit any further transmission of the virus, PHE and West Sussex County Council would like to find out if this variant has spread more widely within the school community.They are asking that your child now takes a COVID-19 PCR test.
"Contact tracing relating to the cases at school has already taken place and these contacts have been isolating.We understand that you might be concerned about this variant and we would like to reassure you that we are following the advice of, and working very closely with WSCC and PHE and everything we are doing is with our duty of care to students,staff and the wider community.There is currently no evidence to suggest that this variant causes more severe illness, or that the vaccines are less effective against it."
You can find out more about booking a Covid PCR test by visiting the Government website here.