Police said the teen was hit in the leg, causing a large, red welt and several puncture marks. The injury did not require medical treatment.
Singalargh was convicted of common assault and offensive behavior following a defended hearing. He had pleaded innocent to the charges.
He was fined a total of $545, of which $389 were paid to his victim.
A more serious charge of assault with a weapon -- the hedgehog -- was dropped. The maximum penalty for that charge is five years in prison.
It was not known whether the hedgehog was dead or alive at the time of the attack, but Senior Sgt. Bruce Jenkins said earlier that it was dead when collected as evidence.