School Security is Homeland Security Grant Act of 2018 This bill amends the Homeland Security Act of 2002 to authorize the use of homeland security grants for enhanced school security efforts. The bill authorizes appropriations for FY2019-FY2023 for homeland security grants, of which not less than 5% in each fiscal year shall be for grants relating to securing early childhood education programs, elementary schools, high schools, and secondary schools. Grants awarded under the Urban Areas Security Initiative or the State Homeland Security Grant Program may not be used to purchase firearms or firearm accessories, such as ammunition, including for use by teachers. Such grants relating to securing early childhood education programs, elementary schools, high schools, or secondary schools may be used for: (1) target hardening and other enhancements of the physical security of such programs and schools; and (2) security planning, exercises, and training.