It sounds like your grandson’s mischievous behaviors have gotten him into trouble this time. Do you think your grandson was bored and looking for things to play with? Or do you think he was trying to steal another person's belongings? These are two very different behaviors that require two different approaches from his parents.
When thinking about the behavior that you are trying to change, focus on the behavior you want to replace it with. For example, if your grandson was looking for toys, you want to work on ways he can deal with boredom or perhaps focus on his asking for permission. If your grandson was attempting to steal, then you want to focus on honesty and respecting other's property.
Ask, but don’t accuse your grandson about why he was in the locker room. One of the most effective consequences for stealing is making the child pay restitution. Now, this doesn't mean he goes out and gets a job to pay for what he took, it means he makes it up to the person he stole from. Have your grandson make a verbal apology, face to face, to the children whose backpacks he was going through. Maybe he should write an apology (with help) to the school for being in a room he wasn't suppose to be in.
Again, this all depends on what your grandson's intentions were by going into the locker room. Only then will you know the correct approach to take when "teaching" him your appropriate expectations.
