Open plan classrooms, where several classes share the same space, have recently re-emerged in Australian primary schools. This paper examined how the acoustics of four Kindergarten classrooms (an enclosed classroom (25 students), a double classroom (44 students), a linear fully open plan triple classroom (91 students), and a semi-open plan K-6 classroom (205 students)) affect speech perception. Twenty-two to 23 children in each classroom participated in an online four-picture choice speech perception task while adjacent classes engaged in quiet versus noisy activities. The noise levels recorded during the task were higher in the larger open plan classrooms compared to the smaller classrooms for both the quiet and noisy conditions. A linear mixed effects model revealed that children’s performance accuracy decreased as noise level increased. Additionally, children’s speech perception abilities decreased the further away they were seated from the loudspeaker, and this effect was stronger the higher the noise level. Children’s response time was also slower in the noisiest compared to quietest classroom. These results suggest that open plan classrooms may not be appropriate learning environments for didactic-style teaching with young children due to their high intrusive noise levels which negatively impact speech perception.