In Chapter 2, we find that Justin is the person who dropped the book:

And seizing Justin by the collar of his blouse, he shook a book out of his pocket. The lad stooped, but Homais was the quicker, and, having picked up the volume, contemplated it with staring eyes and open mouth.

"CONJUGAL--LOVE!" he said, slowly separating the two words. "Ah! very good! very good! very pretty! And illustrations! Oh, this is too much!"

Madame Homais came forward.

"No, do not touch it!"

The children wanted to look at the pictures.

"Leave the room," he said imperiously; and they went out.

First he walked up and down with the open volume in his hand, rolling his eyes, choking, tumid, apoplectic. Then he came straight to his pupil, and, planting himself in front of him with crossed arms--

"Have you every vice, then, little wretch? Take care! you are on a downward path. Did not you reflect that this infamous book might fall in the hands of my children, kindle a spark in their minds, tarnish the purity of Athalie, corrupt Napoleon. He is already formed like a man. Are you quite sure, anyhow, that they have not read it? Can you certify to me--"