Chicago’s elementary school teachers are looking to add half an hour of paid preparation time at the start of their day, a proposal which could potentially shorten the school day for students.

The idea is one facet of the hotly contested teacher prep time issue in contract negotiations between the city’s school district and the Chicago Teachers Union.

Teachers use their preparation periods for grading, conversations with parents and other work outside of class instruction time.

Elementary teachers currently have four one-hour prep periods per week. The city first proposed bumping down elementary teachers to two prep periods per week, and now is proposing cutting them down to three periods. High school teachers would keep their current prep time.

While bell times vary at schools, elementary students are generally in school for about 7 hours. Before 2012, students were in session for six hours or less.

Even with the current system, though, many teachers don’t finish their work at school and still end up taking much of it home.

”Teachers don’t have enough prep time. Period,” CTU spokeswoman Chris Geovanis said.

The lengthening of the school day was a major priority for former Mayor Rahm Emanuel in the 2012 negotiations. Chicago had one of the shortest school days in the country, and Emanuel’s move for a longer day pushed the city closer to average.

”The school day was extended by Rahm — and we don’t believe that by any measure that move has led to greater quality instruction, just a longer and more exhausting day,” Geovanis said. “In the past, before Rahm jammed through the longer school day, the 30 minutes of prep came at the start of the day before students arrived.”

Geovanis added that prep time is a major concern for teachers, and that “people would be striking over that yesterday if they could.”

CPS spokeswoman Emily Bolton said in an emailed statement that “numerous studies point to the importance of increased instructional time, and shortening the school day would be a detriment to student learning and threaten the incredible progress that schools across the city have made.

“Based on educator feedback, we adjusted our original proposal and our current offer respects the role of teacher prep time without shortening the school day,” Bolton said.

CTU President Jesse Sharkey said the union’s proposal to add a 30-minute prep period at the start of the day wouldn’t necessarily mean a shortened school day. He said the district could put students with support staff during those 30 minutes, for example. Teachers are not proposing lengthening the amount of time they spend in school to accommodate the prep period.

“It would be up to them to figure out how to provide that,” Sharkey said. “The length of the school day is ... not something we would be allowed to strike over. So we’re not trying to make proposals that go directly to that. What we’re saying is we are allowed to bargain over prep time, and that’s something which we care much about.”

The CTU announced Wednesday evening that its 25,000 members and another 10,000 school support staff and Chicago Park District workers with SEIU Local 73 would all strike Oct. 17 if their respective demands weren’t met in contract negotiations.