School inclusion of children with Autism Spectrum Disorder (ASD) represents a challenge for all parties involved (family, school and out-of-school actors such as medico-social professionals). To the best of our knowledge, existing tools focus either on the pupil, or the relationships between two parties, always including parents. Considering a systemic approach and knowing that collaboration among stakeholders of school inclusion allows the latter to succeed, we decided to create a tool to enhance and facilitate communication and information sharing among them. For a better usability and to ensure satisfaction of the need expressed by those stakeholders, we used participatory design to create the web-based app ToGather. We used (fictitious) realistic cases to create scenarios that contain tasks to do on the web application, adapted to each participant role (parent, teacher and external actors). We used objective (interaction annotations, website navigation inspection) and subjective (questionnaires) measures to assess the design of ToGather, with regards to usability, user experience, cognitive load and elicited self-determination. Results indicated that ToGather was very usable, with an excellent user experience, and a relatively low cognitive demand. Some design mistakes have been identified and corrected for the enhanced final release.