Research shows that schools are largely a professional solo cultures (Blossing 2014), which impede teachers' professional development as learning takes place in social interaction and through living-practice dilemmas (Wenger 1998/2004). Changing cultures is difficult, not least in the world of education that on the whole has been a solo culture since the establishment of convent schools.The purpose of this study is to investigate whether teachers believe that through professional collaboration in the form of Teacher Learning Community (TLC) has contributed their knowledge and compentence in the field of embedding formative assessment. TLC is a sort of community of practice for improve teaching. The study works according to the hypothesis that “Teachers believe that professional collaboration in the form of Teacher Learning Community (TLC) has contributed to their knowledge and expertise in the field of embedding formative assessment”. The study was conducted at a large secondary school in central Sweden which organized its collegial learning according to TLC and exclusively worked to develop and modify instruction regarding embedding formative assessment during five years before the study. The theoretical approach applied is based on the tradition of "school improvement" with a human relational and group dynamic organizational based on social-constructivism) (Schein, 1994; Giddens, 1984; Wenger 1998/2004; Schmuck & Runkel, 1994; Blossing 2008; Scherp 1998). The study is quantitative and was conducted using a questionnaire, processed through a factor analysis, that is, a multivariate analysis. The analysis was conducted in four stages: stage 1: factor analysis to reduce factors exceeding the value of 1; step 2: categorization of all questions related to the component; step 3: measurement of the homogeneity of issues with Cronbach's Alpha; step 4: hypothesis testing in Person. The correlation was 0,686 (p<0,001). This is a so called census survey and the high response rate gave the study high validity. The study concluded that it is possible to change a school's historic solo culture to a collaborative team culture through systematic collegial cooperation in the form of TLC, and as a result to change the current teaching patterns.