Rep. Jesse Kremer, R-Kewaskum, said a statewide law would clarify the issue and protect individual school districts from lawsuits over their own policies.
"If we put everyone under a statewide policy, the state department of justice will have to back up this law, and take care of this and defend it," Kremer said.
If a transgender student didn't want to use the assigned restroom, Kremer's bill requires school administrators to find an alternative, such as a unisex or faculty restroom.
For Shannon Panszi, the mother of a transgender elementary school student, the bill is discriminatory.
"My children have a high respect for adults, for government, for school, and for them to hear that they're saying, 'We don't think you are who you say you are,' that's devastating," Panszi said.
Her child's Milwaukee County school, which she asked FOX6 News not to name to protect her family, allows transgender students to use the restroom with which they identify.
Kremer said the issue came up in his hometown of Kewaskum last school year, and district officials allowed the student to use a faculty restroom.
Kewaskum Superintendent Jim Smasal said he couldn't discuss the decision, citing student privacy. When asked, Smasal also declined to endorse Kremer's bill.
Administrators in Oconomowoc, faced with similar questions in 2013, decided that transgender students could use a unisex restroom.
Milwaukee Public Schools administrators leave the decision up to individual schools, a district spokesman said.
Kremer said he believed transgender students would face less bullying if they used a faculty or unisex restroom, but said he was also interested in promoting "safety" and "privacy" for heterosexual students.
"It's funny because I'm hearing from the other side, the side that says they support the transgender students, and they feel they should be able to do whatever they want -- but no one on that side is talking about all the other kids. Our bill talks about all kids," said Kremer.
Gov. Scott Walker has endorsed Kremer's legislation, which is currently circulating in the state Capitol for additional co-sponsors.
Panszi said lawmakers should stay out of the debate.
"My child and I think most people go to the bathroom for the same reason -- which is to use the bathroom. They're not there for any other reason," she said.