We evaluated structural stabilities for A-site-ordered perovskites using the computer program "SPuDS", and succeeded in synthesizing designed AMn₃Al₄O₁₂ (A=Y, Dy) under high-pressure and high-temperature conditions. Bond Valence Sum calculations with the refined structural parameters revealed that the compounds are represented as ionic A³⁺Mn³⁺₃Al³⁺₄O^2-₁₂. Mn³⁺ ions at the A' site produce high-spin magnetic moments and they align antiferromagnetically. The A-site Dy spins, on the other hand, behave paramagnetically and show no correlation with the A'-site Mn spins. The antiferromagnetism of the A'-site Mn spins is caused by the nearest neighboring Mn-Mn direct exchange interaction.