SEATTLE — A company that’s come under scrutiny for operating a gravel yard next to an elementary school in unincorporated Snohomish County sent the school’s principal a letter this month warning her and her teachers to stop making public complaints about noise and dust from the yard — or else. The Mukilteo School District hasn’t replied to the cease-and-desist letter from an attorney for Mountain Loop Mine, which lacks permits for the yard by Fairmount Elementary School. But the head of the local teachers union called the letter “shocking” and several legal experts questioned the approach. “Th…