I remember in middle school our coach have us do these workouts/games for us:
1. 75 meter all out sprints with little rest (30 seconds), did about 10 of these, they got pretty tiring, when I was only 11 years old, I thought I was going to die or something!
2. 4-5x800 meter intervals on grass (on some type of soccer, football, lacrosse fields), we did these pretty hard, just fast enough so we could do the whole workout, we wouldn't do this every week. Got about a 3-5 minute rest (we waited until the next group after us got back from their set)
3. Capture the flag, awesome game
4. 6-3-6 workout (if you were a good runner, you would do 6-6-6). This is where you would warm up for six minutes, run all out for three minutes, and rest for 6 minutes, we would usually do only 2 of these)
5. We would always do pushups, situps, crunches, or any other strengthening exercise, do this with the athletes.
6. We would spend about 2-4 weeks of "conditioning" doing these hard workouts about every other day, when the meets came around, we eased back a little. On easy days, we would warm up a mile, and play a game like capture the flag or run around for 15-25 minutes
After looking back on it, these workouts were easy as shit. But it's a good start for many younger runners like I was.