Schools often tolerate noisy classrooms. Managers may find acoustical treatment expensive and difficult, or they may not even recognize the problem. However, poor acoustics interferes with speech intelligibility which influences the teaching and learning process. Speech intelligibility can be described by STI values, which depends on the background noise level and the reverberation in the room. The background noise value used for the calculation is often given as one value for the whole room. However, traffic noise entering the classroom through bad quality windows, or noise coming from the corridor through a door, typically result in an uneven distribution of background noise in the classroom. In this way a bad quality or open door or window acts as a big acoustical bridge and more or less like a surface source with properties of the noise filtered through such a building element. This paper reports results of STI in noise values for the example of a classroom simulated in Odeon® software. The conclusion shows that in certain situations, the distribution of the STI values in the classroom does not depend only on the position of the teacher but also on the position of the receiver related to the location of the acoustically weak element.