School classrooms are enclosed settings where students and teachers spend prolonged periods of time and therefore risky environments for airborne transmission of SARS-CoV2 . While countries worldwide have been pursuing different school reopening strategies, most countries are planning to keep schools open during the whole winter season 2020/21 . This poses a controversial issue: ventilation of classrooms (an essential mitigation factor for airborne transmission) is expected to sensibly decrease due to outdoor temperatures getting colder and regulators going to allow less restrictive policies on windows closure . Moreover, most schools are not provided with mechanical ventilation/filtartion systems to date . Fundamental and urgent questions to be addressed are therefore: to which extent can we contain the airborne transmission risk in schools through natural ventilation only? can we reduce the airborne risk with easy to implement countermeasures like lowering the speaking volume? To answer these questions a theoretical risk model based on the emission rate of viral charge from an infective subject has been developed extending previous models for tubercolosis and influenza . The case of an infective student or an infective teacher in a classroom, as well as an infective teacher with microphone have been investigated and compared with infection thresholds for different group sizes . The model also considers the influence of indoor-outdoor temperature difference on the air exchange rate, which seems to be particularly strong during winter.