There is only one way to insure that she will not get caught cheating again...Teach her to cheat correctly. Writing the answers on her arm is a product of bad parrenting. You should teach her to write them very small on a peice of paper that will roll up and fit inside her pen. Or, Make a song out of the contraband answers. This will give you something fun for the family to do at the dinner table. Maybe you can get one of her friends involved, so they only have to remember half as much, then they can switch tests halfway through. Pick her friend with low self estem and possibly a bad family. This will let you improve two lives, not just one.
Sorry, no more good advice tonight, gotta go teach a youth group!