How do i improve on my last 50m in the 100m?

When i run the 100m i have a really good start and im winning for the 1st 50m, but then everyone overtakes me. I am running against people older than me but if i could just finish well i could stand a chance of winning. Im running 12.45. My coach has said i need to do longer strides. Have you got any drills that could help? also should i being do weights? Im 13 by the way. Thanks