The purpose of this study was to determine breakfast habits, dairy product consumption, and physical activity and their relations with body mass index (BMI) in schoolchildren and adolescents. This cross-sectional, school-based study was performed with children aged 6-18 years. Height and weight were measured, and a BMI z-score was calculated for each child. Breakfast consumption frequency, intake of milk and other dairy products, physical activity habits, and mothers' employment status were assessed via a self-report questionnaire. Multiple linear regression analysis was used to estimate the association between these habits and BMI z-scores. Seven thousand one hundred sixteen children were included, 3445 (48.4%) female, with a mean age of 11.7 +/- 2.7 years (5.8-18.9). Of these, 62.6% had breakfast every day. Boys ate breakfast daily significantly more often than girls (64.5 and 60.7%, respectively; p < 0.001). The percentage of children eating breakfast daily decreased with age (79.1% at 6-11 vs. 52.1% at 12-18 years, p < 0.001). Sixty-four (0.9%) children consumed no dairy products. Milk intake was negatively and significantly associated with BMI z-score (beta = - 0.103, p < 0.001). Cheese consumption and the mother being employed were positively and significantly associated with BMI z-score (beta = 0.517, p < 0.001, and beta = 0.172, p < 0.001, respectively). Children engaging in physical activity had higher BMI z-score values than others (0.22 +/- 0.01 vs. 0.12 +/- 0.02, p < 0.001). Prevalence of overweight/obese was higher among children of working mothers compared to those of unemployed mothers (respectively, 29.3, 23.3%, p < 0.001).