What is the way to erase the sin of writing on the school walls?

Writing on walls without necessity is a reprehensible act and is religiously forbidden, based on the Prophet's (peace be upon him) saying: "There shall be no harm or reciprocating harm." Whoever commits such an act after reaching puberty must repent. If it was done before puberty, there is no sin upon them. However, they must remove any defacement they caused to the wall, whether the act was before or after puberty, unless the wall's owner forgives them. Assaulting public property, such as walls in government schools, is an even greater sin.