Science education communities around the world have increasingly emphasized engaging students in the disciplinary practices of science as they engage in high levels of reasoning about scientific ideas. Consistently, this is a critical moment in time in the USA as it goes through a new wave of science education reform within the context of Next Generation Science Standards (NGSS). We argue that the placement of high demands on students' thinking (i.e., a high level of thinking) in combination with positioning students to use disciplinary practices as they try to make sense of scientific ideas (i.e., a deep kind of thinking) constitute critical aspects of the reform. The main purpose of this paper is to identify and describe the kinds and levels of thinking in which students engage when they are invited to think and reason as demanded by NGSS-aligned curricular tasks. Our analysis of video records of classrooms in which an NGSS-aligned, cognitively demanding task was used, revealed many ways in which the aspirational level and kind of student thinking will not be met in many science classrooms. We propose a way of characterizing and labeling the differences among these kinds and levels of thinking during the implementation of a reform-based biology curriculum. These categories, which focus on two important features emphasized in the NGSS, can help us to better understand, diagnose, and communicate issues during the implementation of high-level tasks in science classrooms.