Motivation is a key factor to understand children's academic achievement and early academic achievement is crucial. Then, understanding this motivation for school at an early age appears critical to reduce the achievement gap between children. To understand motivation in early age, exploring and explaining different growth patterns in first grade would be an important first step, but to date, little research exists (Nurmi & Aunola, 2005). To go further, after showing that there are different growth patterns in motivation in the first year of school for important skills (reading), we focus on a malleable explanation factor: the relationship between teacher and children. This relationship would be a critical lever to improve motivation and close the achievement gap between children. Results indicate the convergence of LCMM and KML to a three latent class trajectories solution. Nearly 80% of children show a high-stable pattern of motivation in reading during primary grade but 20% show a changing pattern. When trying to explain the high-stable group membership versus the changing group: controlling for performances in reading in October, the quality of learning support from teacher positively impacts the probability of the child to belong to the high-stable group. These data enlarge previous works showing variability at school entry in children motivation and those establishing that quality of teacher-child interaction could have an impact on pupils’ motivation to read (Lerkkanen et al., 2012).