Defending Special Education Students and Families Act of 2017 This bill amends the Individuals with Disabilities Education Act (IDEA) to authorize appropriations for grants to assist states in providing special education and related services to children with disabilities. For FY2018-FY2022, the bill authorizes specified amounts. For FY2023 and each subsequent fiscal year, the bill authorizes amounts using a specified formula. The bill also amends the Balanced Budget and Emergency Deficit Control Act of 1985 to modify the discretionary spending limits for FY2018-FY2021. For each fiscal year, the bill decreases the limit for security spending by a specified amount and increases the limit for nonsecurity spending by the same amount. The nonsecurity spending increases are the same amounts as are authorized in the bill for additional funding under the IDEA in those fiscal years.