Hello, Irene --
The magnet test doesn't test the coin's authenticity, but rather or not it is a common 1943 steel cent or 1943 copper cent error. If the coin DOES stick to a magnet, it's a 1943 steel cent worth 10 to 50 cents, while a 1943 cent that does NOT stick to a magnet may be a the valuable copper error worth $85,000 or more.
Good luck,
Josh