I've never faked a serous illness, but I did fake being sick to get out of school many many times.
In 4th grade, I had recently moved to a new location, which meant a new school with new people. Even though I quickly made friends, I really didn't like going. At some point during that year, I kept faking sick with my mom until she realized I just didn't want to go. I ended up not going for a MONTH straight. When I came back, I said I had the stomach flu, but obviously my teacher didn't buy that.
Throughout the next few years of schooling, I just really hated dealing with it, and my mom really wasn't in the condition at the time to deal with me herself (She was suffering from depression at the time). So I would basically say "I'm not going", and that was it.
Even though it didn't seem like a problem to me at the time, my schooling obviously suffered from this. I did end up graduating high school at the same time as my age group, but I feel like I just barely scraped by on that one.
I have also stayed from work while being legitimately sick, but I may have went too many days last year. I came down with a really bad cold, and in my mind since I'm the lowest paid employee, I should stay home instead of getting the other employees sick, which would mean they would miss work.