To reduce the infections of the SARS-CoV-2 virus, several types of body temperature sensors have been used to monitor the access of people to enclosed spaces, e.g. thermal imaging cameras and infrared thermometers. However, these approaches cause the concentration of the people flows in checkpoints, increasing the risk of exposure for the people. Moreover, in buildings where the people are located for several hours, e.g. schools, universities, or industries, these approaches cannot reduce the pandemic risk. So, we have proposed and tested a body temperature sensor network in three schools based on wearable temperature sensors monitored via Bluetooth 5.0 by smartphones and/or custom gateways (with a maximum of about one hundred per gateway and about fifty per smartphone). The data collected are transmitted on a server via the internet. The wearable sensor system is a small-size chip with an accuracy of +/- 0.1 °C that can be inset into a specific rubber bracelet for continuous and real-time body temperature monitoring. The sensor system is waterproof and does not present a display, to reduce battery consumption, whereas the communication system and other components have been integrated onboard. The gateway transmits the data via the internet to a server directly. In contrast, the smartphone app (available for IoS and Android) transmits the data and can display the temperature of all the sensors connected to it. For constant and safe monitoring, the developed software is used to analyse the database present on the server to control the measured temperature and produce warning automatically.