Now and then, teachers run into a child who consistently blames other people - children or adults - for his own misdemeanors. One reason he does this may be that the child, like all young children (and like all adults), yearns for approval. Because the child feels that the adult will not approve if he acknowledges that he did something wrong, he denies it. It's obvious to him that someone must have done it, so he names someone else as the culprit.
If this is the case, it's important that you avoid accusing the child. Instead, you might say, "I see you made a mistake. We all make mistakes sometimes. Next time please remember..."
Avoid confronting the child and requiring him to respond. State the reality in a matter-of-fact tone and move on. If you try to discuss the subject, a child like Bobby will argue with you. Then you'll express disapproval of his behavior, increasing his wish to blame someone else! Simply put the truth into words and change the subject. Say, "Bobby, you need to remember not to touch the big scissors, because they're hard to manage. Are you going to cut some more with these child scissors, or would you like the markers instead?"
Be generous with compliments, as well as cautious when it comes to criticism. The child who feels approved of and warmly appreciated is not likely to fear disapproval so much that he can't accept responsibility for his actions.
Another reason for Bobby's behavior could be that he is not accustomed to accepting responsibility for his actions, if adults close to him have made, and are still making, excuses for him. These excuses can be expressed in a number of ways: "Oh, he's tired." "He's been sick." "His brother always makes him do that."
When this is the situation, be empathic, but encourage personal responsibility. Here are some examples: "I guess you're very tired today, but you need to (do/not do whatever it is) anyway." "I think you're feeling a little bit sick today. It's hard to cooperate (with the rules) when you don't feel well, but you need to (do/not do whatever it is) even when you feel sick." "Yes, I see that Joanna is pestering you, but you hit her. Joanna didn't make you hit her. You're in charge of your arm. Don't let it hit anyone."
A third reason for this type of behavior may be that the child rarely experiences negative consequences for unacceptable behavior because his parents endlessly negotiate with him. Have you ever seen a child sit, whining, while an adult scrambles to find a suitable solution ("Would you rather have this one? That one? Me sit here? Me sit there?")? Kind, cooperative parents and teachers try to be accommodating, but at some point, set limits. And they let the child know that there are consequences for unacceptable behavior.
In this case, be fair, but make it clear that something less than desirable results from inappropriate actions: "Listen to my words. You can choose to keep on whining, or you can choose to go over there. But you can't choose to stay where you are and whine. It bothers my ears." Then you need to enforce what you say.
Finally, this behavior may occur because the child has been so over-controlled at home--and maybe at school, too--that he doesn't feel in control of his own actions. This child expects others to discipline him, so he isn't developing self-discipline. In order to learn to control his impulses, a child must be given a certain amount of freedom to govern himself.
If you determine that this is the reason, give the child more control of himself and his learning community. This includes enough chances for him to make meaningful choices; enough opportunities to discuss plans and problems that affect the group; enough invitations to participate in making significant decisions; and allowing enough time and guidance to help the child process mistaken behavior and to see the wisdom of controlling his own impulses.
When a child blames others for his own inappropriate behavior, as you say Bobby often does, I would think of each of the possible causes mentioned above in relation to the individual child to determine which ones fit. I would then tailor my work with him accordingly.