Every public school F1and every grant-aided school shall be open to pupils of all denominations, and any pupil may be withdrawn by his parents from any instruction in religious subjects and from any religious observance in any such school; and no pupil shall in any such school be placed at any disadvantage with respect to the secular instruction given therein by reason of the denomination to which such pupil or his parents belong, or by reason of his being withdrawn from any instruction in religious subjects.