Heat oil in a deep-fryer to 360 degrees F (182 degrees C).
Whisk flour, corn flakes, baking powder, salt, pepper, and sugar together in a shallow bowl. Pour milk into a separate shallow bowl.
Gently press one tomato slice into the flour mixture to coat on both sides and shake off any excess. Dip into milk, then press again in flour mixture. Repeat with remaining tomato slices. Place breaded tomato slices in a single layer on a plate while breading the rest; do not stack.
Cook each breaded tomato slice in the hot oil until golden brown, about 3 minutes per side. Remove to a paper towel to drain and cool.