What will the results of this incident be helpp?

Me and two boys were messing about so we threw an orange and hit his �45 shirt and he snitched... The boys mum came into school last night to talk to the head teacher And had a word about the incident and the head likes me as a pupil so she told me that i might have to pay 1/3 of that what will happen? (also the head teacher said that the shirt stained)