Unfortunately, cutting in various forms has become a wide-spread behavior among teens, especially young girls. Any form of cutting is considered a "Self-injurious behavior" (SIB).  SIB's are not considered recreational, casual or frivolous, but rather a matter of serious concern.  Most often, self-injury is an indication that the individual is attempting to deal with some underlying problem that is causing mental or emotional stress or pain. So, yes, it is wise for you to be worried. But the fact that you know your daughter did this is also cause for hope. Knowing that she cut herself opens the opportunity to get her the help she may need to learn how to deal with life's stress in ways that are healthy and productive.
 
The other good news is that there are professionals familiar with treating SIB's.  I concur with those who advise getting professional help.  You might start with your daughter's doctor. He or she can recommend providers in your area who treat teens. Your doctor may know those who specialize in treating SIB's.  
The professional you choose can help uncover any underlying issues that may be troubling your child by conducting a clinical assessment re: the implications of your daughter's having cut her hand.
 
As the parent, you will want to know if this cutting event is a one time occurrence or whether it is indicative of some chronic way your daughter has been coping with stress. Twelve year old girls, on the threshold of adolescence, often experience a great deal of social and emotional stress. The professional may discover that the "A" itself has some meaning that may lead to a deeper understanding of what is bothering your twelve year old daughter.
 
You may also want to consider a counselor for yourself to help you cope with the concerns that you may have around this issue.  A professional can help you with strategies for dealing with your daughter's behavior, as well as with your own stress level regarding this situation. Overall, consider that you now have an important opportunity for your daughter and/or yourself and other family members to learn more effective and healthy ways to manage emotions and the stress of life.
 
Bette J. Freedson, LICSW, LCSW, CGP
 
The good news is that you know about this incident.