Open plan classrooms, where several classes are in the same room, have recently re-emerged in Australian primary schools. This paper explores how the acoustics of four Kindergarten classrooms [an enclosed classroom (25 children), double classroom (44 children), fully open plan triple classroom (91 children), and a semi-open plan K-6 "21st century learning space" (205 children)] affect speech perception. Twenty-two to 23 5-6-year-old children in each classroom participated in an online four-picture choice speech perception test while adjacent classes engaged in quiet versus noisy activities. The noise levels recorded during the test were higher the larger the classroom, except in the noisy condition for the K-6 classroom, possibly due to acoustic treatments. Linear mixed effects models revealed children's performance accuracy and speed decreased as noise level increased. Additionally, children's speech perception abilities decreased the further away they were seated from the loudspeaker in noise levels above 50 dBA. These results suggest that fully open plan classrooms are not appropriate learning environments for critical listening activities with young children due to their high intrusive noise levels which negatively affect speech perception. If open plan classrooms are desired, they need to be acoustically designed to be appropriate for critical listening activities.