This is what my son’s religion book teaches about Mary. He is in 3rd grade, so it may be a little simplistic for a 12 yr old.
"God gave to Mary all the holy graces that he gave to Eve, and many more. Unlike Eve, she did not fall into sin. In fact, she did not have any stain of sin at all. Mary was full of sanctifying grace, even before she was born…Her soul was perfectly pleasing to God in every way. She was “filled with grace”…Gabriel knew she had never offended God by the smallest sin. Gabriel knew she was “full of grace.” …Mary said “I am the handmaid of the Lord. Be it done unto me according to thy word.”
What I tried to explain to my son when we were studying the Imaculate Conception was not only that Mary was obedient to God when she accepted His Will, eventhough it meant that she would have certainly faced terrible disgrace from her community, but I also wanted to convey the difference between Mary and Eve.
Eve, through her disobediance, brought about the end of paradise and the stain of original sin which became the inheritance of mankind. Mary, through her obediance, became the vessal for the salvation of man, that was lost to us through the sin of our first parents.
Now, about Mary’s free will, I would explain it this way; Mary could have said no, yes she was in a state of santifying grace that would have presumably prevented her from denying the Will of God, but she had to be the one to say yes "be it done unto me according to Thy Will. She accepted God’s Plan for her through her own free will, unlike Eve.
As far as God already knowing that Mary was going to say yes. We could also say that God also knew that Eve would say yes to Satan, that the paradise He created for her and Adam would be forever lost to this earth when she made that decision. That is why the lesson of Mary and the Immaculate Conception is so powerful. A great gift was given to us by God through his son Jesus, and another gift was given to us by Mary when she accepted her destiny as the Mother of the Saviour of the world.
What would have happened if Mary had said no? I guess technically you could say that the Saviour would not have been born, at least not to Mary, but I’m sure the Lord in His Mercy would have made other arrangements (not to be litehearted about it) However, as ridiculous as that sounds, I would not leave your students with the impression that Mary’s acceptance of God’s will was insignificant or somehow not relevant because God made her so that she could not have refused. Her acceptance of God’s Will is the very thing that makes her what she is. In essense, she set right what Eve made wrong in the garden.
Jesus, who was both human and Divine (unlike Mary, who although was in santifying grace was not divine) also had free will while human on earth. He also accepted the Will of His Father by dying on the cross. Once again, an act of love and obedience to God, brought salvation to the world.
