A Comenius project from 2013 to 2015 was successfully completed and all five participating schools wanted to renew the experience with another project. We independently financed a meeting together in Hatvan, Hungary, in order to discuss the possibilities and decided to invite the EcoSchools association to join us in our new project. So, the participants were 5 high schools and the Netherlands branch of an international association, Eco Schools within the FEE organisation, called SME Advies.The aim of the project was to accompany SME in their development of an app for tablet and smartphone to be used by schools which are members of the EcoSchools organisation. The app would also be available to non-members and free to download from the App store and Google Play. EcoSchools encourage work on sustainability in schools and award flags and certificates to reward progress. The app developed is a tool to enable schools or other organisations to collate data and create a sustainability profile at the beginning and end of the school year or period. The data can be analyzed and consulted by schools so they can compare their progress before and after their work on sustainability and compare with other schools or countries. The 5 high schools collaborated on all the steps in the development of the app with the developer in order to uniformise the diagnostic test procedure, to make it applicable or usable in a maximum number of countries. None of the high schools had previously participated tin the EcoSchools programme so all signed up to their countries' organisations for the period of the project. Students involved began by doing the diagnostic test for their schools using the paper questionnaires provided. This was the basis for adapting the process to a digital format. Once each school had completed the tests, the students joined forces to select the questions suited to the new format and worked on the rewording of them. Some questions were altered or rethought to make them as compatible as possible to a wide international public.Students worked on 2 levels - first as members of Eco committees in schools following the steps to reach the EcoSchools objectives and secondly as testers and sounding boards for the app at every stage of its development. Between the student exchange meetings and transnational meetings, skype meetings between the teaching staff and SME Advies staff were held regularly to compare progress and discuss the steps undertaken. Students also prepared diverse presentations for the workshops and conferences held during the exchanges.The app went live in Autumn 2019 and was officially presented at the EcoSchools National Operators Meeting to attendees from 60 countries in November 2019.