Could everyone please put your head down on your desk?
ーCould everyone please put your head down on your desk while I'm writing on the board?
「黒板に書いている間、机に頭を伏せてください」
to put one's head down on one's desk で「机に頭を伏せる」と言えます。
to write on the board で「黒板に書く」
ご参考まで！
カナダ 
