Sergio,
If RT must be used to meet a standard for, as you stated, classroom acoustics, then RT must be used for the room acoustic design and RT must be measured to verify performance.
The relative meaning of RT for small rooms is the issue for debate. A good acoustician will know that achieving the target RT does not necessarily ensure that the acoustics will be appropriate for the room's intended purpose. If the RT for an untreated classroom is calculated to be, say, 2.5 seconds, then the appropriate steps should be taken to reduce the RT to the required value. If the design target is, say, 0.8 seconds, then absorption should be included in the design to ensure that that target will be met.
Having said that, other calculation tools and professional expertise should be used to ensure that the resulting classroom with its RT of 0.8 seconds actually sounds good and meets a requirement for good teacher-student communication.
Example: Achieving the target classroom RT may be as simple as adding carpeting the floor. While this would result in meeting the design target, it would be an irresponsible approach. The carpet on the floor will do nothing to minimize acoustical anomalies that may result from ceiling and wall reflections. It is the responsibility of the acoustician to ensure that the required amount of absorption is distributed appropriately in the room to not only achieve the design target, but also to minimize acoustical anomalies that are not predictable with RT equations.
HTH.
