This paper highlights specific aspects of high-school students' reasoning while coping with a modeling task of plant growth in a computer-supported educational environment. It is particularly concerned with the modeling levels ('macro-phenomenological' and 'micro-conceptual' level) activated by peers while exploring plant growth and with their ability to shift between or within these levels. The focus is on the types of reasoning developed in the modeling process, as well as on the reasoning coherence around the central concept of plant growth. The findings of the study show that a significant proportion of the 18 participating dyads perform modeling on both levels, while their ability to shift between them as well as between the various elements of the 'micro-conceptual' level is rather constrained. Furthermore, the reasoning types identified in peers' modeling process are 'convergent', 'serial', 'linked' and 'convergent attached', with the first type being the most frequent. Finally, a significant part of the participating dyads display a satisfactory degree of reasoning 'coherence', performing their task committed to the main objective of exploring plant growth. Teaching implications of the findings are also discussed.