In an abrupt shift in teacher contract negotiations, School Superintendent Carol R. ­Johnson retreated Wednesday from a sweeping proposal to add 45 minutes to Boston’s elementary, middle, and K-8 schools, and instead will pursue a narrower effort that would add two hours at a handful of low-performing schools.

Extending the school day had been a hallmark of ­Johnson’s push to secure substantial changes in the contract in an effort to reinvigorate city schools. But after two years of intense and exhaustive talks with the teachers union over a new contract, Johnson said it appeared that her proposal was going nowhere. The union had insisted that teachers be paid at a rate she said the city cannot afford, prompting her to pull the 45-minute proposal.