Why is Anne grounded from attending the church picnic?

Marilla believed that Anne had taken her brooch an wouldn't admit it. She was told tto stay in her room until she confessed.

"You'll stay in this room until you confess, Anne. You can make up your mind to that," she said firmly.

"But the picnic is tomorrow, Marilla," cried Anne. "You won't keep me from going to that, will you? You'll just let me out for the afternoon, won't you? Then I'll stay here as long as you like afterwards cheerfully. But I must go to the picnic."

"You'll not go to picnics nor anywhere else until you've confessed, Anne."