I find that high Ab is usually flatter than the other notes around it, but you can try adding the middle and ring fingers on the right hand. This fingering is more stable and slightly more flat than just the left hand.
Not THAT middle finger, the other one Also maybe experiment with your embochure, changing the direction in which the air is entering your flute (try blowing down, more than across). Avoid covering the hole with your lip.
Also maybe experiment with your embochure, changing the direction in which the air is entering your flute (try blowing down, more than across). Avoid covering the hole with your lip. 