SPRINKLE gelatine over milk in small saucepan; let stand 1 min. Cook on low heat 3 min. or until gelatine is completely dissolved, stirring constantly. Add sugar; stir until dissolved. Cool slightly.
PLACE yogurt, ricotta cheese and lemon zest in blender; cover. Blend until smooth. Gradually add gelatine mixture, blending well after each addition. Pour into medium bowl; cover. Refrigerate 30 min. or until slightly thickened.
STIR in strawberries. Refrigerate an additional 30 min. or until mixture is of mounding consistency. Top each of 12 graham squares with 2 Tbsp. of the strawberry mixture; cover each with a second graham square. Repeat layers once. Refrigerate until ready to serve. Store leftover desserts in refrigerator.
