This article interprets the perceptions and the reasons that induce teachers to make their decisions to support immigrant students in real class context, while taking into consideration the needs of other students. Based on the teaching work analysis approach and the concept of managing the unexpected, the article aims to explore the real work executed by teachers by virtue of classroom factors, and to search the reasons behind the actions they take in their classes. Drawing on qualitative interpretive methodology, data were collected through class observation, audio recording, and interviews with two teachers and their three immigrant students from two different highly diverse primary schools in Quebec. Findings show that teachers opt for collective, small group and individual support to cover their students' needs when they anticipate or discover students' difficulties. More specifically, the analysis of teachers' reasons shows that implementing and processing of individual teaching support practices are influenced by the perceptions and the decisions that teachers must make momentarily when they encounter a student difficulty. Teachers' decisions result from an inner negotiation process. They have to choose quickly between implementing supporting practices and maintaining their ongoing prescribed tasks. More particularly, the teachers adopt four scenarios to manage the unexpected in their classes: 1-integrating the unexpected into the teaching activity, 2-considering the unexpected as a disruptive element, 3-exploiting the unexpected for the benefit of the class, 4-transferring the responsibility to the student.