In this paper, we present a comprehensive IoT solution for monitoring air quality in classrooms, which can also be applied to indoor environments in general. The hardware IoT sensors are designed with a modular approach, allowing components such as the connectivity module (e.g., Wi-Fi) and sensors to be easily replaced or upgraded for different purposes. The IoT platform follows a Publish/Subscribe (Pub/Sub) messaging model, supporting scalability, extensibility, and decoupling devices from services. This design simplifies the integration of new devices and services, enabling seamless plug-and-play functionality for new modules. We manufactured the sensors and deployed the system in a real-world setting to monitor classroom air quality at Hanoi University of Industry.