Theft at school

There have been numerous instances of theft while my child was either at Aftercare or during class hours. Through this I have accepted that I am unable to control all of this but with help from the school and joint responsibility of parents we will curb this behaviour in the school. Regardless of how many time I confront Mrs Lopez I am unable to get her and the school to acknowledge or help. There have been other children who we have personally witnessed complaining about theft and still no acknowledgement. I am now unable to accept that anything may go missing from my child’s bag without the school even looking into it or acknowledging it. The excuse that Mrs Lopez uses is that my 6 year old child should be more responsible with her belonging - this said when we have found her belongings missing from her class where she has nor removed them. Surely there must be some process or due diligence to be taken by the school when they are made aware of an issue (on more than 1 occasion, by more than one learner)