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.