The soon-to-launch NASA James Webb Space Telescope (JWST) will provide unprecedented opportunities for characterizing exoplanet atmospheres. However, given the relatively large amount of observational time required to study the atmospheres of potentially Earth-like exoplanets, it is important to develop an understanding of the likely performance of JWST to best allocate telescope time and to select promising targets. Here, we study the ability of JWST to identify atmospheric constituents of hypothetical Earths transiting 8 different M-type stars. Our approach pairs atmospheric chemical composition predictions for these worlds to a full-physics radiative transfer model to generate high-resolution transit spectra. We then explore spectral feature detectability using a novel simulation tool for James Webb — the JWST Exoplanet Transit Simulator (JETS). Critically, JETS ingests planetary and stellar spectra to then simulate how JWST, and its myriad instruments and observing modes, would observe a given planet in transit. We validate JETS against other existing JWST simulation tools. Using our models, we explore the number of observed transits required to detect key atmospheric species across a broad range of wavelengths, instruments, and modes for our grid of coupled exoplanet and host star scenarios.