Does anyone have any tips on running a 50 metre sprint?

It's for my school sports day. Last year I came 2nd in the 75 metre sprint. How can I improve my speed? Everybody laughs, because they say I run like a 'retard', its quite funny, but how can I improve my posture? and should I? Because my coach showed me a little something at the end of today's lesson, but it made me run a bit slower.. Sports day is in two weeks. Thanks