Before revision:  As expected, we find that the SOT decreases with ventilation quality. We also establish the recommended "vacancy time" after an activity in the room. For example, to achieve an airborne transmission risk of less than 5\%, the pre-pandemic ASHRAE recommended ventilationwould allow 37-minute lessons followed by 35-minute breaks but if the ventilation is poor, 25-minute lessons should be followed by 166-minute breaks. Our model also uncovers power-law relationships for the concentration as a function of time and particle emission rate and for the TTI as a function of ventilation rate and particle emission rate;
After revision:  As expected, we find that the `time to probable infection' (TTPI). Good agreement with CFD models is achieved. Furthermore, we derive power laws that quantify the effect of ventilation, emission rate and for the TTI as a function of ventilation rate and particle emission rate;