What is an average shot-put throw for a high school boy?

If you're about 18 years, i would say 12 meters, if you've not had any javelin training. Given some technique and fitness training, you'll reach 15 meters pretty quick.

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You have your numbers horribly messed up. Freshman to Sophomore ranges from 70(untrained)-150 (medium to well trained) feet on average (21 meters-45 meters). As a senior, you can expect throws to be 160+ feet on average, some top of the line seniors get up into the 200's and the farthest IRL throw i have seen is 225 feet (68 meters).