You need to attend all classes for each module. There is a clear correlation between those who do not attend and those who fail. In some modules, the assessment methods make it very difficult to pass or achieve a good mark without attending. Lecturers will be taking registers of attendance.
If you are absent from class for medical or other reasons, you should inform your course administrator and, if necessary, supply documentation to cover your absence, e.g. a sickness certificate from your doctor. Once you return you should make sure that you seek any necessary assistance from tutors and fellow students in making up the missed work. Should you be persistently absent from the course, you may be denied reassessment in the event of failure and your registration with the University may be terminated.