The school is considering toughening the attendance rules to reduce student lateness.

toughening 是动词 toughen 的现在分词形式。toughen 由形容词 tough（严格的、强硬的）加上后缀 -en（使变得……）构成，意思是“使变得更严格或更强硬”。toughening 在这里指“加强、使更严格”的动作。