Many junior and high school students are interested in studying particle physics or astro(particle) physics through their researches, sometimes within school activities and sometimes not. Measuring cosmic rays are one of the easy ways to provide such opportunities without any accelerators or beam facilities. However, in most schools or teachers, preparing and handling detectors may be significant obstacles. CosmicWatch is a USB bus-powered cosmic ray muon detector with a plastic scintillator and a SiPM, which we have adopted and distributed among Japanese school students for their activities. CosmicWatch records the ADC values corresponding to the pulse height of triggered events, and its spectra are very useful in extracting qualified cosmic muon events. In order to do so we need to well understand the spectral feature or components in advance and such studies can also be students' researches. In this paper, we report several studies focusing on the spectra obtained by CosmicWatches -- for example, the response dependencies on the interacting position of the scintillator, or induced particle types. These findings will be useful for many students in the future.