Classroom acoustic design is typically guided by reverberation-time–based criteria, assuming that global decay adequately predicts speech intelligibility. This field study examines whether edge-based acoustic treatments influence speech intelligibility thresholds under steady background noise beyond conventional decay metrics. Four geometrically comparable primary-school classrooms were investigated: one acoustically optimized ceiling reference, one untreated control, and two rooms equipped with edge-based systems (primarily absorptive vs. primarily reflective/scattering). Room impulse responses were measured at multiple source–receiver configurations, and standard parameters (STI, C50, D50, T30) were derived. Psychoacoustic sentence-repetition tests with children (7–9 years) under calibrated noise conditions were used to determine speech-level and SNR thresholds at a predefined sentence-repetition intelligibility criterion. Both edge-based treatments improved STI, C50, and D50 relative to the untreated room, while affecting T30 differently. In the psychoacoustic evaluation, untreated conditions required the highest SNR thresholds. Both treated rooms showed reduced SNR requirements compared to the untreated state. Although not all SNR comparisons reached statistical significance after correction for multiple testing, effect sizes indicated consistent directional improvements. This study was designed as an exploratory pilot investigation to assess the plausibility and directionality of potential effects rather than to provide definitive statistical confirmation. The findings suggest that intelligibility under noise is not fully explained by reverberation time alone and motivate further investigation of structural room-acoustic characteristics in small classrooms.