Wireless sensor networks (WSN) are a major technological innovation developed in recent years with many important applications, for example in Cyber-Physical Systems and Internet of Things environments. Although there are many proposals for handling real-time traffic in WSNs, most of them fall into best-effort mechanisms that do not provide real-time guarantees. At the MAC level, the IEEE 802.15.4 standard defines a reservation mechanism called Guaranteed Time Slots (GTS) that uses a first-come first-served service policy, which is not enough to ensure the real-time delivery of packets. In this work, we present a formal definition of the scheduling problem for a sensor network using a reservation mechanism (such as GTS) that provides guarantees for the real-time delivery of packets. We also present a heuristic approach for finding feasible schedules when possible or determining if the requirements of a set of sensors are not feasible.