The article describes a tool to solve the problem of insufficient development of schoolchildren’s arithmetic operations skills. The authors believe that the use of computer technologies in the process of teaching mathematics will allow the building of individual learning trajectories for each student and ensure the development of arithmetic operations skills to a stable state. The authors introduce a new concept of “skill architecture” and demonstrate the possibilities of building the skill architecture of a school mathematics course: structuring skills into groups and creating a new approach to practicing mathematical skills from simple to complex, starting from elementary school. The article presents the rationale for the development, concept, and requirements for an online simulator for practicing the skills of performing mathematical operations. The concept of the simulator is based on the results of research: analysis of the subject area of the school mathematics course; identification of the architecture of skills that allows building individual learning trajectories; development of a methodology for evaluating the effectiveness of tasks for practicing a particular skill by analyzing learning curves. The online application “Mathematical Operations Simulator” has been developed. The concept of the proposed online mathematics simulator can be used as a basis for developing similar solutions for any subject area of the school curriculum. The simulator can also be used in distance learning conditions.