Combine sugar, cornstarch, and water in a heavy saucepan; stir well.
Peel and slice 6 peaches; add peaches to sugar mixture. Cook over medium heat, stirring constantly, until mixture is thickened and bubbly. Cool. Stir in almond extract.
Peel and slice remaining peaches. Arrange peach slices in pastry shell. Pour glaze over peaches, coating well. Chill.