ST. PAUL — Teachers at St. Paul Public Schools went on strike Tuesday after last-minute efforts to reach a contract agreement failed.

Union members hoisted strike signs and picketed outside the city’s public schools. The walkout — the district’s first strike since 1946 — cancelled classes for roughly 36,000 students and forced parents to make alternate plans for their children.

The school district and the union that represents about 3,550 teachers and support staff negotiated for six straight days and, despite a mediation session that lasted until 3 a.m. Tuesday, were unable to come to terms on an agreement.

“I can assure students, families, staff members and our community that the Board of Education, my team and I did absolutely everything we could to avoid a strike by teachers,” Superintendent Joe Gothard said in a statement.

The St. Paul Federation of Educators planned to picket at all 67 public schools across the city, according to union spokeswoman Megan Boldt.

“We wanted to settle this contract and be in school with our students Tuesday morning,” said Nick Faber, president of the Saint Paul Federation of Educators. “Unfortunately, after more than nine months and marathon bargaining over the weekend, district leaders weren’t willing to move on the issues educators and parents know will help students thrive and break down racial barriers in our schools.”

The union’s priorities include hiring more mental health professionals, multilingual staffers, special education teachers and restorative practices specialists.

“I want to make it clear: I believe our students need and deserve additional support. That has never been in question,” Gothard said. “However, we must prioritize our spending because we have limited resources. We need to place new investments where they are needed most. This is what SPPS proposed as a responsible way to increase student support and remain accountable.”

School district officials said creating mental health teams at every school alone would cost around $30 million a year.

After an 11-day strike last fall, the Chicago teachers union touted contract language that guaranteed nurses and social workers would be assigned to every school. The Chicago school district committed to hiring an additional 250 nurses and 209 more social workers by 2023 and put $500,000 toward recruiting and training people for those jobs and other roles.

The demands by striking Chicago teachers for more nurses, librarians and social workers highlighted concerns that resonate in high-poverty school districts nationwide, where shorts of support staff leave educators feeling stretched.

Unions and professional groups for such employees say those jobs often are lower priority when budgets are tight, but their absence can have profound effects on student learning and teachers’ work.

As part of St. Paul Public Schools’ strike plan, classes were to be cancelled Tuesday and Wednesday, but breakfast and lunch will be served at some community and school locations and 24 public schools will serve meals.

Students from grades six to 12 will be allowed to keep their school-issued iPads and access online academic resources. And starting Thursday, the district will open seven sites for up to 4,000 elementary school-aged children, where they could be supervised.