The Internet of Things is considered as the next big technology revolution after the invention of the Internet. Data of IoT are very useful and could produce such huge creativity for us to build many useful applications in our daily life. In Cambodia, the IoT is still a new technology for research and development. However, Security and Privacy are the main key issues for IoT and they are still facing some enormous challenges such as DDoS Attack on the centralized server, data leakage, and some IoT device products are a weakness in authentication, management and access control that is an opportunity to hackers can control their devices. So, the main objective of our research is to investigate how Blockchain and Smart Contracts can contribute to building trustworthy, secure and privacy-aware IoT systems. In this paper, we proposed a system architecture model of using a Blockchain called Hyperledger to secure the Internet of Things network in a smart classroom, and we developed the IoT platform for a smart classroom that including the mobile application as an end-user to remote or control devices in classroom, Blockchain and Smart Contracts of Hyperledger as the method to security and privacy for smart classroom, LoRa as a network for long-range communication and low power consumption IoT devices, The Things Network as a public LoRa server to communication over cloud server. After we investigated on Blockchain and Smart Contracts of Hyperledger to security and privacy for a smart classroom system, the results showed that it is very useful application for secure and privacy-aware IoT systems such as disintermediation, robustness, traceability, enhanced security, transparency, prevented DDoS attack on the centralized server, protected data leakage, eliminated a central authority, prevented dishonesty, data manipulation and prevented data fraud.