Asperger. I was 14 y.o starting high school in my country when I saw this asperger kid who could get away with anything just because of his illness. He was almost untouchable and the teachers would always defend him and stand by his side, even when HE was the one starting troubles. They would always give him more opportunities and stuff. So, considering that at the time I wasn't motivated to befriend with my schoolmates (most of them were like this lol: https://www.theclinic.cl/wp-content/uploads/2013/07/flaitehermoso.jpg) and didn't care at all about what people would say, I thought "well, why not pretend to be autistic and take some advantage of it then?". And I did, I fooled them all.
It was all like a master plan and I put a lot of work in this specific lie. I made a lot of research on asperger syndrome and autism, and then I began to act as If I had a mild form of asperger, enough to make teachers see me as one. I stopped making eye contact and I began to speak with a monotone voice and did a lot of weird things to make it more credible. I went to the school psychologist and behave as one too, then went to a hospital psychologist and got her to make a report in which she stated that my psychological profile fits with asperger symptons, and this report was sent to my school. Every teacher started to treat me differently and they gave me free pass to do the exams whenever I wanted. I had more freedom and I enjoyed it. After some time, the hospital psychologist began to suspect and of course, I disappeared and she never saw me again. Hell, I even visited some neurologists and would always leave before any test or evaluation. I was discovered by a psychiatrist one time.
I remember I confessed it like two years ago to a friend of mine and we would laugh a lot when teachers gave me free grades and free pass to do whatever I wanted. And well, when I turned like 17 y.o I got a bit tired, so I dropped the facade and some students resented me for lying like that for years. But the teachers continued to believe it so I kept taking advantage for as long as I could. I remember one time when I considered the idea of faking epilepsy and/or speech impediments, but I thought that was way too much. Anyway, this lie didn't prevent me from having girlfriends, so it didn't backfire that much, thankfully.
I apologyze for my grammar, English isn't my first language, so you'll notice a lot of mistakes in my writings.