St. Andrews has been preparing for two different options: distance learning or a hybrid model with students learning both on an off campus. The school had planned to make a final decision the week of Aug. 10 and notify families, according to a note on the school website.
Trump argues that children are being harmed by being away from the classroom. Federal medical experts have said decisions about reopening schools should be made locally.
Maryland Gov. Larry Hogan criticized the Montgomery County order, saying those decisions should be made by schools and parents, not politicians.
The White House and St. Andrew's Episcopal School did not immediately return requests for comment late Saturday on the Montgomery County order.
Associated Press writer Brian Witte in Annapolis, Md., contributed to this report.