The article is devoted to the development of criteria as to safety study load for the school-age children. The main methodological approach is to determine the influence of the factors of the educational process on the development of mental fatigue in children. In the 1092 school-age children we evaluated independent associations between educational load levels and the development of mental fatigue. It was established that the mental capacity and the mental fatigue are influenced by factors like the severity of subjects, the number of lessons per week, homework assignment time duration (the factors contributing is from 37,0 to 55,3 %; p < 0,05–0,01). The calculated risks of developing the severe mental fatigue that are influenced by too high level of the criterion indicators of the study load are 1,39–2,09 (p < 0,05–0,001). By developing predictive models was proven the school-age children proportions reduce with severe fatigue on 9 % with 5 lessons a day instead of 6–7. We developed specific hygienic recommendations for safe level of the quantitative and qualitative study load components, including homework assignment time duration which can be used to develop sanitary regulations for schools. The maximum permissible study load for the 1st grade students is 20 hours per week, the 2-nd – 21, the 3-rd – 22, the 4-th – 23, the 5-th – 27, the 6-th – 28, 7–9-th – 30, 10–11-th – 32. Homework assignment time duration is 1 hour for the 2–9-th grade students and 1,5 hours for the 10–11-th grade students. It is of current interest to review the content of the State Standard of Secondary Education in order to reduce the children’s perception of subjects as difficult, as well as to reduce the number of academic disciplines in the timetable up to 8 for junior students, 10–13 for middle and senior students. We developed and implemented an algorithm for calculating the mental fatigue assessment integral indicator for monitoring the school-age children’ organism functional condition. This will allow us to optimize the distribution of the study load during the school day and the week and improve the organization of the educational process in general, that will help preserve and strengthen school-age children’ health. The perspective of further research is to establish the optimal level of motor activity of students and the influence of lifestyle factors on the school-age children’ health.