Since defining "scrotum" so upsets such librarians as the prurient Ms Koch at the French Road School in Brighton (a suburb of Rochester, New York), let's help her out of the woods.
"Scrotum," says the trusty AMERICAN HERITAGE DICTIONARY is "the external sac of skin enclosing the testes in most mammals." As for "testis," the singular of "testes," it is "the male reproductive gland, the source of spermatozoa and of the androgens, normally occurring paired in an external scrotum (which is, you might tell those Heritage people, a tautology because a scrotum is always external) in man and certain other mammals."
The Dictionary might also want to add that the "certain other mammals" are male by definition.
If librarians can't handle this, we're sure that the 4th graders of Rochester can!