Similar word to underachieving:

backward, birdbrained, defective, dim, dim-witted, dopey, dull, dumbbell, dumbo, dumdum, dummy, exceptional, feeble-minded, gorked, half-witted, held back, imbecile, lamebrained, mentally defective, moronic, numbskull, opaque, pinhead, retardo, sappy, simple, simple-minded, slow, slow-witted, stupid, subnormal, touched, underachieving, weak, yo-yo