The cheers build to a crescendo as Brendan Driscoll approaches the wall at the completion of his 500-yard freestyle race. The support might seem odd to the uninitiated, since he’s typically last in his heat. Until he pulls himself out of the pool. Then, the explanation is as clear as the sparkling waters he has just emerged from. An eighth grader on the Manchester-Essex High swim team competing against able-bodied athletes four years his senior, Driscoll does not have a leg below his left knee.