SAN FRANCISCO (CBS SF) — Monday is first day back to school in San Francisco and the possibility that students could be shipped right back home in the next few weeks due to a teacher’s strike is very real.

San Francisco Unified School District Superintendent Richard Carranza says he’s hopeful that won’t happen.

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“We’re still negotiating,” Carranza said. “We’re still negotiating. We’re very optimistic that we are going to be able to resolve the contract.”

From teachers, the loudest cry is over money. Their union — United Educators of San Francisco — is demanding a 22 percent pay raise over three years.

The skyrocketing cost of living in the city has forced teachers to live outside and commute in. Teachers say more time on the road means less time for students.

“When they can’t live here, when they spend their time on the road, instead of spending their time with the kids. They have to get away to beat the traffic instead of being able to volunteer for an after school program,” union president Dennis Kelly said. “That damages the child, that damages the school, that damages the relationship.”

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But Carranza says a 22 percent wage hike will take from other programs.

“We can’t fulfill a 21 percent salary increase over three years without decimating the funding at school sites,” Carranza said. “I don’t think there’s one teacher or one paraprofessional in this district that wants to take money from the classroom.”

The school district is offering 8.5 percent over three years.

Mayor Lee did want to tell KPIX 5 what pay raise, if any, he thinks teachers should get.

“Oh boy. I’m not in a position to weigh in on that,” Lee said. “Probably would create more problems if I did.”

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The union is especially concerned with raising the pay for teachers’ aids, which make $25,000 a year.