Preheat the oven to 375° F.
In a large saucepan, combine the peaches, 1/3 cup of the sugar (5 1/3 tablespoons), lemon juice, cornstarch and cinnamon; toss to coat the peaches.
Place the pan over medium heat and bring to a boil.
Cook until the mixture thickens (about 1 minute).
Remove the pan from the heat and transfer the mixture to an 8-inch square baking pan.
To make the topping: in a large bowl, combine the flour, baking powder, the remaining tablespoon of sugar and salt.
Work in the margarine with a fork until the mixture resembles coarse crumbs.
Add the milk and stir until the flour mixture is evenly moistened.
Drop 8 tablespoons of the topping mixture onto the peach mixture.
Bake until the topping is golden brown and the filling is bubbly (about 20-25 minutes).
Cut into 8 pieces and serve.