A group of parents in San Francisco are rallying around a second-grade teacher with breast cancer who was required to pay for her classroom's substitute teacher while she is on medical leave.

The parents at Glen Park Elementary School were stunned to learn that the teacher's paycheck was being docked $195 for each day she is out ill with the disease.

"I just feel sad that from what I heard, she is a very good teacher and I just feel sad what's going on to her," Narciso Flores-Diaz, a parent, told NBC Bay Area on Wednesday. "Our school is pulling together to help her and to make her feel that she’s not alone."

Teachers in the San Francisco Unified School District get 10 sick days per year, and then another 100 days of extended medical leave time.

But after a teacher uses the 10 sick days and is on the extended leave, the district bills them for the cost of a substitute, in keeping with a little-known provision of California's education code.

Glen Park Elementary School in San Francisco. Google maps

Parents at the school have raised more than $13,000 to cover the substitute's pay.

The popular teacher, who has asked the media to withhold her name, also had another backstop — her union's bank of donated sick days for members in need.

"This is not unique to San Francisco. This is not a district-only rule," San Francisco Unified School District spokeswoman Laura Dudnick said in a statement to NBC News on Thursday.

Susan Solomon, president of United Educators of San Francisco, said the union might take on this issue during the next round of bargaining talks: "As always, we look forward to making improvements in this and other parts of the contract."

Some parents and at least one key lawmaker said they'd like to see a change to this previously, little-known paragraph of the California Education Code.

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Years ago, California teachers opted not to pay into the state's disability insurance program, so they can't draw from those benefits. So lawmakers in 1976 inserted into the education code that a teacher who couldn't work due to illness or injury must pay for his or her substitute.

State Sen. Connie Leyva, a Democrat from suburban Los Angeles and chair of the body's education committee, vowed to change that law.

"Candidly, I think that times have changed and it’s our job to change with the times," Leyva told NBC Bay Area.

The 17-year veteran teacher at Glen Park deserves community support, according to a GoFundMe page established for her because "she has nurtured our children and now it is time for us to take care of her."

"Her dedication and love for her students" can’t be overstated, according to the fundraiser website. "Just a few days after her surgery, she took the time to write out 22 completely personalized notes to the students in the class thanking them for their support, telling them she missed them dearly and encouraging them to continue working hard."