He is in Miss Li's office,he is helping Miss Li (clean) the desks and board.
他正在李老师的办公室里,帮李老师擦桌子和黑板.办公室有黑板吗？不对吧！填benches更合适吧：长条椅board其实除黑板以外，还有写字板、告示牌的意思