Well, the university I attend insists that if you have time off, you must be given special permission by the department (I had to provide a doctor's note when stayed at home with dd as she had chickenpox!) However, I am in a subject which usually has about 200 students in each lecture and I have known people who don't bother to attend lectures at all, so you might get away with it if you can catch up with the work. Personally, I wouldn't do it. I'm studying maths, and a week off means missing approx 8 lectures and 4 seminars. Depends on lots of things really, but you should be able to find a student handbook or something to download from the university website, to see what is expected of you.
If all else fails, you could make up an excuse, which won't require a doctor's note or anything!!