The Fermi Large Area Telescope (Fermi-LAT) has been widely used to search for
Weakly Interacting Massive Particle (WIMP) dark matter signals due to its unparalleled sensitivity
in the GeV energy band. The leading constraints for WIMP by Fermi-LAT are obtained from the
analyses of dwarf spheroidal galaxies within the Local Group, which are compelling targets for
dark matter searches due to their relatively low astrophysical backgrounds and high dark matter
content. In the meantime, the search for heavy dark matter with masses above TeV remains a
compelling and relatively unexplored frontier. In this study, we utilize 14-year Fermi-LAT
data to search for dark matter annihilation and decay signals in 8 classical dwarf spheroidal
galaxies within the Local Group. We consider secondary emission caused by electromagnetic cascades
of prompt gamma rays and electrons/positrons from dark matter, which enables us to extend the
search with Fermi-LAT to heavier dark matter cases. We also update the dark matter subhalo
model with informative priors respecting the fact that they reside in subhalos of our Milky Way
halo aiming to enhance the robustness of our results. We place constraints on dark matter
annihilation cross section and decay lifetime for dark matter masses ranging from 103 GeV to
1011 GeV, where our limits are more stringent than those obtained by many other high-energy
gamma-ray instruments.