Please check which travel restrictions are still in effect. The official U.S. Visas News and the NAFSA's Visas and Entry are good source.
As of March 2, 2021
F1 students traveling directly from UK, Ireland, and Shengen area are exempt from the 14 day travel restriction. You don't need a special letter. You need a valid I-20 and valid F1 visa to enter the U.S. J1 students need to consult with the nearest US embassy/consulate for the exemption before departure.
F1 and J1 Students traveling from Brazil, China, Iran, and South Africa need to spend 14+ days in other areas before entering the U.S.
F1 and J1 Students crossing the border from Canada and Mexico to attend school (CalArts) have an essential reason to enter the U.S. No special letter is needed.
COVID Test Requirement for U.S. Entry
In order to enter the United States, you will be required to present a negative COVID-19 test, taken within three (3) calendar days of departure, or proof of recovery from the virus within the last 90 days.