Several properties of Earth's magnetic field (field vectors, time dependence) are measured in various locations using a smartphone/tablet magnetic sensor. To enable a proper use of the magnetic sensor as a classroom tool, the exact location of the sensor in the device and its resolution must be identified in a first step. Then, students may perform several measurements on their own, allowing a comparison of the recorded field vectors and a discussion of the possible deviations. We also show that long-time measurements of the local magnetic field are possible using the smartphone sensors, which can be compared to available internet data. All the information obtained enables a better understanding of the properties of the Earth's magnetic field and of the requirements for real applications of magnetic sensor elements.