Isolation: you are sick and avoiding any contact with others in your household and the public.
Quarantine: you have been in contact with a person who is sick and are avoiding contact with the public and ceasing all non-essential activities to reduce the risk of transmission to the public.
Close contact: Someone who was within 6 feet of an infected person for a cumulative total of 15 minutes or more over a 24-hour period starting from 2 days before illness onset (or, for asymptomatic patients, 2 days prior to test specimen collection) until the time the infected person is isolated
CDC released alternative quarantine recommendations just after Thanksgiving. You can see those here. The CDC maintains that the most effective way to reduce transmission is to adhere to a full 14 day quarantine, however, this can create hardship. Their research showed an "acceptable risk level" if quarantine were reduced to 10 days or 7 days with proof of a negative COVID test 5 days after exposure.
SC DHEC left the decision as to how to implement contact quarantine to the school districts. CCSD did not adopt the 7 day quarantine, so the minimum quarantine for a close contact is 10 days from date of last exposure to the person with COVID.
Isolation guidelines have not changed. A person must be in isolation for 10 days from the onset of symptoms or 10 days from the date of testing for an asymptomatic person.
When to test?
DHEC recommends PCR testing as soon as possible after being notified of being a close contact. Early testing helps with contact tracing and early isolation precautions. If the test is negative, it is recommended that you re-test 5 to 7 days after the last date of exposure. If you test negative via PCR on day 5 to 7, that is considered "diagnostic" and your risk of developing the illness beyond that is minimal.
Early testing may yield a negative result if the virus has not replicated enough to be detected by the test. The PCR test, however, is extremely sensitive, so even if you have a low viral load it can detect the virus significantly better than a rapid antigen test. If either test is positive at any time point, the sensitivity is such that a positive antigen or PCR test is considered diagnostic and you are COVID positive.
What's the difference between PCR test and rapid antigen test?
Basically, PCR is better and considered the gold standard. Rapid antigen tests are pretty good for determining if you are infected (positive) but are woefully inadequate in detecting the virus if you are not symptomatic, so not considered diagnostic if your test result is negative.