If you plan to travel outside the United States, the CDC offers recommendations for a safe return to the U.S. and school:

Get tested 3-5 days after travel AND stay home and self-quarantine for 7 days after travel.
- Even if you test negative, stay home and self-quarantine for the full 7 days.
- If your test is positive, isolate yourself to protect others from getting infected.​​

 (10 days isolation from start of symptoms or test date, whichever occurs first)
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If you don’t get tested, it’s safest to stay home and self-quarantine for 10 days after travel.​

Avoid being around people who are at increased risk for severe illness for 14 days, whether you get tested or not.

Here's the link to the travel restrictions and recommendations: Requirement for Proof of Negative COVID-19 Test or Recovery from COVID-19 for All Air Passengers Arriving in the United States | CDC