(CNN) A San Francisco teacher who is on medical leave has to worry about more than just battling breast cancer.

On top of footing medical bills, she has to pay for a substitute teacher at Glen Park Elementary School.

All teachers receive 10 paid days of medical leave a year in the San Francisco Unified School District . If a teacher needs more time, they can get another 100 sick days -- at a price. Under a 1976 California law , the cost for the substitute teacher will be deducted from the teacher's salary.

The average daily cost of a substitute teacher in San Francisco is $203.16, San Francisco Unified School District spokeswoman Laura Dudnick said. In 2018, the average teacher salary in the school district, excluding benefits, was $82,024.37.

"This reflects California Education Code language related to extended sick leave that applies to all other school districts in California," Dudnick said in a statement. "This is not unique to San Francisco. This is not a district-only rule."

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