Why I'm not underachieving is simple - I don't get the chance =P
Seriously though, if I get so much as a B, my mum starts bugging me about how I need to do better, what's wrong, am I feeling okay, would I like to talk, etc. I can't just go, Oh, I'm just a bit tired/I just messed it up/(whatever) doesn't stick in my head, as then she'll either tell me she's going to start cutting off the internet at 7pm again, or she'll get on the phone to my teachers and get them to talk to me (This has happened before O_o)
The other part is, if I don't do well, not only will she never stop nagging me about it, I'll also never hear the end of it from both my sister and everyone at school =/ Which is a good motivator. Plus, tiny tiny classes make a huge difference (compare how much attention and help you currently get with what you'd get with only 14 people in your year =P and only 7 or so in some lessons too)