A new contractor had to be found following the collapse of Muirfield Contracts in March.
Angus Council has confirmed the appointment of Robertson and a cost hike of 2.7% to £12.6m to build the Timmergreens and Warddykes schools.
Finance spokesperson Councillor Alex King said: “The overall project costs have increased by £347,000, taking the gross cost to £12.6m.
“The majority of this 2.7% increase results from construction inflation between the original and revised tender, and is a direct result of the delay caused by having to replace the contractor following the collapse of the original construction company.”
Education spokesperson Councillor Sheena Welsh said: “It is great news that we’ve now got a contractor in place for this first phase of our major investment programme to upgrade Arbroath’s primary schools.
“We now expect the project to be completed around a month later than expected.
“The proposed completion date for Timmergreens Primary School is now May 2016 with Warddykes Primary due for completion the following month.”