Given the worldwide proliferation of cyberattacks, it is imperative that cybersecurity education and training are addressed in a timely manner. These activities typically require trainees to do hands-on practice in order to consolidate and improve their skills, for which purpose training environments called cyber ranges are used. In this paper we present an open-source system named CyRIS (Cyber Range Instantiation System) that supports this endeavor by fully automating the training environment setup, thus making it possible for any organization to conduct more numerous and variate training activities. CyRIS uses a text-based representation in YAML format to describe the characteristics of the training environment, including both environment setup and security content generation. Based on this description, CyRIS automatically creates the corresponding cyber range instances on a computer and network infrastructure, for simultaneous use by multiple trainees. We have evaluated CyRIS in various realistic scenarios, and our results demonstrate its suitability for cybersecurity education and training, both in terms of features and performance, including for large-scale training sessions with hundreds of participants.