Advancing Student Services In Schools Today Act or the ASSIST Act This bill establishes programs and requirements to support the provision of mental health and substance use disorder services at schools. Specifically, the bill provides for a 90% Federal Medical Assistance Percentage (i.e., federal matching rate) under Medicaid for services that are furnished by mental health and substance use disorder providers at schools or school-based health centers. Qualifying services include those provided by school counselors, social workers, and psychologists. The bill also establishes a grant program to increase the number of mental health and substance use disorder providers at schools, colleges, and other educational institutions.