缺课 is a separable verb (离合词) meaning 'to miss a class' or 'to be absent from a lesson'. The verb 缺 and the object 课 can split when inserting 了, 过, measure words, or time duration between them, e.g. 缺了三次课 ('missed class three times') or 缺过一个星期的课 ('missed a week of class'). It always refers to a specific class session or course, not general absence from school.