Undernutrition causes long-term damages on the physical and cognitive development and half of deaths among children under 5 y. Most of the current interventions are concentrated on improving nutrition among young children and mothers. However, malnutrition is also prevalent among older children and adolescents, especially in Africa and in South-East Asia. It has adverse effects on their global development because of the high requirements in energy, protein and micronutrients resulting from growth and brain development spurts occurring these periods of life. Our research evaluated the prevalence of malnutrition and their determinants factors among school-aged children and adolescents in Senegal and Cambodia, as well as the effectiveness of micronutrient fortified food in schools in Cambodia. A cross-sectional study was conducted on a representative sample of ~ 600 children aged from 5 to 17 years from primary state schools of Dakar area, selected through a two-stage random cluster sample (30 schools × 20 children). Elevated rates of thinness (19%) and micronutrient deficiencies (iron 39%, iodine 33%, zinc 26%) were showed. The food consumption survey revealed insufficient micronutrients and energy intake, as well as contribution to total energy intake insufficient in proteins and excessive in lipids, which reveals poor conditions among school children in Senegal. In Cambodia, a cluster-randomized placebo-controlled double-blind trial was conducted among 2400 children aged 6-16 y to measure the impact of fortified rice consumed during 6 months within the school feeding program of WFP on their micronutrient status and cognitive performance. Before the intervention, poor cognitive performance was associated to stunting, iron deficiency and parasite infestation. The growth was positively associated to zinc status, which was negatively associated to parasite infestation. Rice fortified with vitamin A improved vitamin A status and rice with the highest iron concentration had a positive impact on cognitive performance. Data from Senegal and Cambodia were analyzed to study indicators of malnutrition among school-aged children and adolescents. According to the WHO recommendations, we suggested mid-upper-arm circumference (MUAC) cut-offs to screen for acute malnutrition among children above 5 years in Cambodia (15.5 cm, 16.4 cm and 18.2cm in boys and 15.4 cm, 16.6 cm and 17.9 cm in girls aged 5-7.9y, 8-11.9 y, and 11-13.9 y). We confirmed the need to correct plasma concentrations of retinol-binding-protein and ferritin according to inflammatory status, and showed the interest of a similar correction for transferrin receptor, these corrections improving the estimation of prevalence of vitamin A and iron deficiencies in populations where inflammation is endemic. Undernutrition remains a public health issue among school-aged children and adolescents in Senegal and Cambodia. Determination of malnutrition in this age range could be improved by research on indicators taking into account intra variability populations, environmental factors and specific needs of these populations. School-based interventions like the one in Cambodia have positive effects on nutrition and development of children, advocating for extension of school feeding, including in disadvantaged urban areas. Distribution fortified food in existing school canteens is a cost-effective strategy to improve nutrition and health. More research and efforts should be shifted to the nutritional status of adolescent girls because of the risk of early childbearing, still commonly practiced in developing countries, and the importance of adequate nutrition during pregnancy. Investing in nutrition in this age range is an opportunity to consolidate the progress achieved in pregnant women and young children through a life-cycle approach and to optimize the development of next generations in developing countries.