The study presents the results of an Action-Research project carried out during the COVID-19 pandemic with Italian teachers of primary, lower and upper secondary schools, interested in monitoring the activity of students in that difficult situation. The purposes of this study were: (a) to demonstrate that the involvement of teachers in the creation of metacognitive tools promotes the use of formative assessment at school; (b) to verify to what extent the use of a metacognitive form makes students more aware of the mistakes made during the test. The results were: (a) teachers showed great enthusiasm in adapting the metacognitive form to their school subject; (b) students pointed out high percentages of appreciation of the form; what is more, a group of students improved in identifying the typology of errors and understood more clearly what they should study in a better way to correct their mistakes; another group noted that their awareness of their strengths in study method had grown; finally, one group highlighted that the skills used during the completion of the form were also useful in other areas of their daily life, not only at school. Both teachers and students appreciated the online version of the tool: the pie charts created automatically by the system, by displaying percentages over typologies of errors made, provided immediate feedback, motivating students more and more. The study shows how much reflection on mistakes can be a source of growth.