Many adolescents have poor eating habits. As a major part of their caloric intake takes place at school, the present study aims to examine the effect of increasing the availability of healthier foods in school canteens on sales, student attitude and self-reported behaviour. A quasi-experimental study was carried out at two vocational schools in the Netherlands over a 10-month period, where the visible share of healthier products was gradually or abruptly increased from 60% to 80%. Outcome measures were sales data for healthier and less healthy foods and drinks in the canteens, as well as surveys. The proportion of healthier products sold increased from 31.1% during the baseline period to 35.9% in the final period. A gradual increase led to higher relative sales of healthier products (40%) than an abrupt change (34.5%). Survey data showed that students’ moderate satisfaction remained insensitive to the changes over time. Overall, results suggest that increasing the availability of healthier products in school canteens leads to small positive changes in sales of products, particularly in the product groups beverages and sandwiches. A gradual introduction may ensure that students slowly get used to assortment changes.