why quarantine for 14 days if tested negative?

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
Should I self-quarantine if I have symptoms after testing negative for COVID-19
If your test is negative and you have symptoms, you should continue to self-quarantine away from others for 14 days after your last exposure to COVID-19 and follow all recommendations from the health department. Additional medical consultation and a second test may be needed if your symptoms do not improve.
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 important to quarantine for COVID-19 even if I get a negative test result?
This is important because symptoms can appear up to 14 days after you've been exposed and are infected. A negative result before the end of your quarantine period does not rule out possible infection. Additionally, you do not need a repeat test unless you develop symptoms, or if you require a test to return to work.
Full answer in: www.cdc.gov