Well, I doubt it's "defective", no coin would leave the mint in that condition. It could be a counterfit or some kind of joke or gag item (compare the back with other pennies under a magnifying glass).
The most simple explaination is the one Mel gave, it sounds to me like someone was having fun with the machine tools in shop class.
The only reference I found to "clipped pennies" (pennies that have their copper scraped off) is from an article about the American Civil War.
French and local lawyer David Roer repeatedly warned the Republican faithful to be on watch for the dreaded "copperheads" -- those Democrats and anti-war advocates who took to wearing a clipped penny on their lapels as a mark of their political belief.
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