Some good stuff in this thread.
A lot of sprinting is just trying to get all of your movement into a nice rhythm. You get some kids who have none but eventually they will find it. This is what I like to do throughout a season. We are always working on form. I try to keep it simple because of the amount of groups I need to work with so there are folks that are way more complex.
I like the Foot over Calf drill myself. It's a really good visual example and its something during practice where you can say Foot over the calf! Kids react to it.
We will do a ton of high knee marches. No landing on the heels! After they master this we migrate to high knee runs at something like 1/4 or 1/2 speed. But we always go back to the high knee marches. Part of our diet.
Wickets are great but not necessary. I'll lay down tape marks over 90ft. in a bunch of lanes based on stride length, speed, height, etc. Early season I have the kids just run on the actual tape. After they have mastered this I lay a little strip of 3/4 inch plywood on each tape and the kids will run in between each strip. It's an obstacle so kids will still get the knees up/feet up. Wickets are expensive and cumbersome and I would need at least 60 so plywood strips!
Every so often you can continue on with the drill even in the absence of the tape/wicket/wood. So if it extends out for 35m have them continue on running for another 20-25m or however long you want. Have them imagine they are still going over the sticks. 2-3 months of this and you will have some really good looking sprinters
For the torso I dont mind of a kid has a bit of a forward lean but obviously the more forward they are the more it restricts the full range of motion of the hips and the knees coming up. Practice this on yourself. You will feel it.
To help improve the posture I will often times ask my kids at the end of a the last few reps of a distance to just put their arms up.
If they are doing some 300s on the baseball field I'll lay a cone out at 225-250 and when they hit that cone they put the arms up. Try this on yourself. You can't run fast but if you focus on getting the knees up you will find it so useful. You will feel that it's ridiculously hard to have any backside action.
Hills. Hills will naturally help them get the knees up
Lastly I like to do some high knee marching with your arms up.