How Can Teenagers Best Grasp Saints and Angels?

There is a very fine, brief booklet by Fr. Paul O’Sullivan, O.P., called All About the Angels. That would be an immense help to you for a complete but accessible presentation. I recommend you read over that and have the high schoolers read it if you can, and they will learn a great deal. The Catechism of the Catholic Church also has clear teaching about the angels that confirms Fr. O’Sullivan’s more complete treatment.