As someone with asthma myself, I've never used it as an excuse. When it happens badly, you really know about it - the last time I had a proper attack was after shearing alpacas (don't ask, really). I spent the night unable to get any kind of full breath, finally gave in at 8am and took myself to the doctors, by which point I was almost grey and could only talk 2 words to a breath. Crying wolf on that one would be fairly pointless, surely? Besides, when I was at school there was always headaches to use, or even better, PMT...