Background: School is an important setting to promote healthy eating habits for children. We compared the effects of 4 conditions on the nutritional quality of snacks brought to school. We also investigated teachers' opinions regarding the process, barriers, and facilitators in regulation implementation. Methods: We compared the nutritional quality of morning snacks based on 1958 photographs from schools divided into 4 conditions: regulation based on a list of permitted foods and beverages; regulation banning sweets, chips, and sugary drinks; information without regulation; or no intervention. Based on 5 focus groups (N = 18 participants), we investigated factors influencing regulation implementation. Results: In schools with a list of permitted foods and beverages, 76% of the children had a healthy snack, compared to 52% to 54% in the 3 other conditions (p < 0.01). They also brought less sweet foods compared to those in the other conditions (41% vs. 68% to 71%, p < 0.01). In focus groups, most teachers supported regulation but also expressed ambivalence about their legitimacy. Conclusions: In this study, a regulation based on a list of permitted foods and beverages showed the best results. All actors should be involved in a progressive implementation process to increase acceptance of such regulation.