Teachers usually work alone during lessons. Supervision over students in the classroom or at school is a joint activity, though. It is impossible to create a complete, consistent, meaningful and complementary educational process, which executed by a dozen-odd teachers without a well-structured interaction between them. This may involve a need to coordinate the scope, and order of issues discussed during the lesson, methods used as well as the implementation of joint projects. From the organisational point of view, it may be crucial to provide time and space for joint meetings, during which group activities are planned and carried out, and after their completion, analyses are carried out is to help improve future steps. The expectation that teachers will work together in the planning and implementation of educational processes is a requirement for state schools. This requirement reflects the belief that in today's world the success of societies is largely determined by the ability of its members to collaborate creatively and to self-manage. In order to teach, teachers themselves need to work together.

The paper will include results of research on various aspects of cooperation between teachers: teachers involvement in the work of task forces operating at schools, planning, implementation and analyses with respect to conducted educational processes, mutual support, problem-solving, self-evaluation of work, interaction of teachers working with one class, and the organization of work of teachers at school, concerning analyses of the educational process. The analysed data comes from an external evaluation of schools, carried out in three consecutive years, between September 2010 and June 2013, within the Education Supervision System. The analyses contain data obtained from studies in which a total of 27,675 teachers participated, as well as 680 headmasters of various profile schools.

Data was collected using quantitative methods (online survey), as well as qualitative ones (FGI, IDI). In data analyses, the following variables were used:

• School type (elementary, junior high, high),
• School size (small - up to 200 students, average: 201-400, large: more than 400 students),
• Location type (rural, rural-urban, urban).

The results of analyses carried out indicate that despite a high level of cooperation between teachers declared as both informal and formal (teamwork), it serves primarily bureaucratic purposes. Collaboration between teachers is best seen in related subjects, both at school and one-class teacher level, regarding mutual assistance in the problem-solving process. Differences in co-operation between teachers appear primarily to be caused by the type of school (elementary, junior high, high), but neither the size nor school location is associated with different forms and objectives of cooperation.