why 14 days quarantine is needed?

Does the student need to go through 14 days of quarantine even if the test results for COVID-19 were negative?
To best protect the student and others, have the student stay home for the full 14 days. Check your local health department's website for information about options in your area to possibly shorten this quarantine period. (See When to Quarantine for more information.)
Full answer in: www.cdc.gov
How long do I have to self-quarantine if I have COVID-19 but don't have symptoms?
If you continue to have no symptoms, you can be with others after 10 days have passed since you had a positive viral test for COVID-19. If you continue to have no symptoms, you can be with others after 10 days have passed since you had a positive viral test for COVID-19. Most people do not require testing to decide when they can be around others; however, if your healthcare provider recommends testing, they will let you know when you can resume being around others based on your test results.
Mar 12, 2021
Full answer in: www.cdc.gov
Why do you have to quarantine for 14 days after travel during the COVID-19 pandemic?
You may have been exposed to COVID-19 on your travels. You may feel well and not have any symptoms, but you can be contagious without symptoms and spread the virus to others. You and your travel companions (including children) pose a risk to your family, friends, and community for 14 days after you travel. May 3, 2021
Full answer in: www.cdc.gov
Why is it necessary to take the COVID-19 test 3 days before arriving in the United States?
The 3-day period is the 3 days before the flight's departure. The Order uses a 3-day timeframe instead of 72 hours to provide more flexibility to the traveler. By using a 3-day window, test validity does not depend on the time of the flight or the time of day that the test was administered.
Full answer in: www.cdc.gov