Critical thinking skill is very important, because it can help us to think rationally to find alternative solutions for a problem. The demand for critical thinking in the field of education is stated in the objectives of the 2013 curriculum. Therefore, critical thinking can be trained during studying at school especially in mathematics learning which is inseparable from the role of teachers to improve students’ critical thinking skill. Ennis (2011) states that critical thinking is a logical or sensible thinking focuses on decision making about what to believe or do. Problem solving is part of critical thinking. It is an attempt to find a way out of the difficulties. Polya (1985) propose 4 steps to resolve the problem; understanding the problem, planning problem-solving strategies, implementation of problem-solving strategies, and looking back all conducted steps. This research uses critical thinking skill indicators by Ennis (1996) called FRISCO (Focus, Reason, Inference, Situation, Clarity, Overview).Teachers need to have critical thinking skills and receive training in terms of content knowledge and pedagogical skills in order to help students acquire critical thinking skills in teaching programs.Ashton (1988) stated that the greatest obstacle to have critical thinkers at schools is that teachers have yet to possess critical thinking knowledge and skills. To train students to have more participations, good questions, able to identify estimates and priorities, able to infer various persuasions in schools, teachers should be train according to these skills (Aktas, 2012).

Based on the high performance teacher selection guidelines set by the government in 2018, High performance teachers are teachers that have performed above standard set by educational unit, covering pedagogic, personality, social and professional competencies, produce creative or innovative works recognized at the regional, national and/or international levels, and directly guide learners to reach achievement in the intracurricular and / or extracurricular fields. According to the researchers, high performance teachers are teachers who have superior skills compared to other teachers and have a very good performance so as to reach the achievement. Therefore, this study aims to describe the critical thinking skills of high performance mathematics teachers in solving problems. The subjects in this study was a junior high school teacher in class IX SMP Negeri 2 Mojoanyar who was awarded as an high performance teacher.

The method used in this research was descriptive qualitative. Thus, this research analyzed and described the critical thinking skill of high performance mathematics teacher in solving mathematical problem. The critical thinking skill of the teacher was obtained from in-depth interviews based on the task of solving mathematical problems. The mathematics problem solving task was “How many sheets of A4 paper can be obtained from a piece of paper sized 1 m²? It is known that the standard size for A4 paper is 210 mm x 297 mm. Try revealing your strategy to get the most A4 paper produced from paper."

Before being used, the task was consulted to the supervisor and tested to three participants: 1) tutor of junior high school mathematics olympiad, 2) an undergraduate student of  Mathematics education program with the best GPA in her class, and 3) Doctoral students of Mathematics education program. The participants 1) and 2), give wrong answer, the most A4 paper produced from paper sized 1 m² is 16 sheets obtained from division of 1 m² paper area with A4 paper area, but this is not possible. While the participants 3) give answer 12 sheets. Compared to the three participants, the high performance mathematics teacher who is the subject of this research has the best skill by answering closest to right that is 13 sheets. For more details, in completing the mathematical problem-solving task, the subject of high performance mathematics teacher shows the critical thinking skills as follows;

Stage 1:Understanding the problem, teacher focused on reading the problem,beside getting information according to the text, she found other information developed from the problem, the first about the form of A4 paper in this problem is full paper which not obtained from the result of the remaining cutting paper joints. The second is a paper sized of 1 m² not only 1m x 1m, but also 2m x 0.5m and 4m x 0.25m.

Stage 2:Planning problem-solving strategies, in the reason indicator, the teacher's idea to solve the problem was illustrate through A4 paper cutting sketch of a paper of 1 m² and used the concept of integer operation. In the inference indicator, the teacher believed that illustration sketch of A4 paper cutting was appropriate strategy because it could help find the number of A4 paper cuts close to the real context. Although the question was related to the area of ​​the rectangle, she did not use this concept because it was the wrong strategy. She chose to use the concept of integer operation because the precision was required so that the pattern of A4 papers being made did not exceed the paper area of 1 m².

Stage 3:The implementation of problem-solving strategies on situation and clarity indicator, teacher had applied the idea from the planning stage, but there was a developing strategy. She set the pattern of A4 paper pieces with different positions to fill more paper area of ​​1 m² so the A4 paper produced was 13 sheets.

Stage 4: looking back all conducted steps, in the overview, teacher re-examines the suitability of the information, the solution steps and the results of its work. According to the information on the problem, the teacher believed the answer was correct because the strategies had been tried, the A4 paper produced the most is 13 pieces. From the results of the research described above, it is known that the critical thinking skills of high performance mathematics teachers in solving mathematical problems satisfy FRISCO critical thinking indicators well. So, further research related to the high performance mathematics teachers such as PCK, MKT, or teaching skills for teach Critical Thinking is interesting to be conducted. The results of this study are expected to increase knowledge that supports the progress of education, especially teaching mathematics.