Objective: The Healthier Options for Public Schoolchildren (HOPS) intervention aimed simultaneously to change school policies regarding the nutritional content of school-provided meals, nutrition and healthy lifestyle curricula, and other school-based wellness activities. Researchers examined how HOPS affected students' academic achievement, attendance and behaviour. Design: Longitudinal study of 23,335 elementary-school students aged 4-16 years enlisted in kindergarten to grade 8 in the 2005-2006 school year and followed through until the 2008-2009 school year. Setting: HOPS was implemented in 11 Buffalo (New York) public elementary schools at the start of 2007-2008 school year and concluded in January 2009; 34 other district elementary schools were chosen for comparison. Method: Administrative records were obtained containing student demographics, mathematics and English test scores, and attendance and disciplinary records. A difference-in-differences approach was used to measure the HOPS intervention's effect on mathematics and English test scores and the proportion of days present or not suspended. Significance was tested at 1% and 5% levels. Results: Analysis comparing students in intervention and comparison schools revealed an increase in standardised mathematics test scores among all students (p < 0.05), with particularly strong impacts on girls (p < 0.01), African American and Hispanic students (p < 0.01) and economically advantaged students (p < 0.01). HOPS had an adverse impact on attendance and disciplinary referrals (p < 0.01), although the effects were small. Conclusion: When considering the implementation of a school-based wellness programme, administrators should consider its academic benefits. Research into the effectiveness of programmes such as HOPS at improving cognitive and non-cognitive outcomes of school-aged children is particularly relevant given recent US Department of Agriculture rollback of whole grain, sodium and milk requirements.