You can discuss with your partner about what you would do if you came across such cases - would you report it to the teacher or not. You can also talk about the reasons why this classmate started stealing from other classmates, whether it was because he/she is poor, or just a thief. Finally, if it is because he/she is poor and wants to help their family/wants to eat because they have no money, you can discuss how you can help them, perhaps everybody could chip in and give them some money, or somebody could bring an extra lunch to school so that your friend would eat.