This work describes the results of speech intelligibility tests and measurements performed in one primary school laboratory-classroom before and after a sound adsorption acoustical treatment. It consisted of placing an acoustically absorbent suspended ceiling with the central part reflective, covering the upper part of lateral and back walls with mineral wool panels, covering the lower part of the walls and the back corner with plasterboard panels. The occupied mid-frequency reverberation time (RT) decreased from 1.6 s to 0.4 s. The different classes, of grades 2, 3, 4, and 5 students, turned in the lab-room to carry out the tests in presence of different types of noise. Speech bubble and fan-coil noises were emitted into the room, traffic noise came from an external source and impact noise was generated on the floor upstairs the classroom. A sample of 210 students aged 7 to 11 years were tested before and after the treatment. A total of 3360 tests were administered to the children. The correlations between speech intelligibility and objective parameters are obtained for children of different age. The availability of data referred to two different conditions, with high and low RT respectively, allows to determine intelligibility decrease due to reverberation. Results from a similar school survey were also compared.