This paper reports the development and validation of a digital application (Assessment Scale of Oral Language at School -- Decision Support System, EVALOE-DSS) for the professional development of teachers aiming to improve their students' linguistic competence. We performed a multiple case study involving seven teachers and their students from seven different schools. Over a six-month period, the teachers used the digital application to self-assess their classes, make decisions and introduce changes into their teaching practices. The researchers observed four class sessions of each teacher and assessed them with the same digital application. Each teacher-researcher dyad discussed the tool and its use over three meetings during the semester. Despite some differences between the teachers, use of the digital application produced progress in the competences of both the teachers and the students. Thus, the digital application can be used for professional development in teachers.