He said he is pushing it because of recent passage by California of a law that allows transgender students to choose for themselves which restroom they should use and with which gender of sports teams they participate.
He said he is not aware of any problems with the issue so far in Utah schools.
Brandie Balken, executive director of Equality Utah, said the proposal is "at least misguided and at worst cruel."
She said, "To require parents to provide proof of fetal type of a child to participate in school … is an invasion of privacy."
It is inappropriate, Balken added, to isolate children over gender-identity issues.
Kennedy's bill redefines "gender" in state law as "the male or female phenotype" of an individual as documented by a birth certificate or a letter by a doctor "based on a physical examination of the person's genitalia."
The legislation adds that a school should make available "reasonable alternate-bathroom accommodations" if a student requests such accommodation.