Please don't do this unless you want to push them further away from you.
They will not "get" the lesson or principle you are teaching. It will just distance them more from you and give them justification to lash out more at you and disrespect you. They are sad and hurting. Where the heck is their father in all this?? Maybe all these boys want for Christmas is a father in their life that loves them and cares about them. Do you have family around? Do these boys have an extended family support?(for you also)
Kids will act out and be disrespectful to mask what they are feeling deep down. Most often it is because they are scared,angry and feel bad about them self. They will turn to drugs, sex partners and other households to find the acceptance they so desperately are looking for.
I bet you are trying your best with the cards you were dealt and the cards you willingly chose to play. These boys need lots and lots of love. I would put up a Christmas tree. I would tell them that they are making cookies with me tonight and we are taking them to some families in our community who are struggling right now. I'd jack up the stereo with Christmas music and we'd have fun making cookies together. I'd ask them what they'd like for Christmas dinner and have them help me make it in the kitchen. This situation will be better remedied with more love,fun,laughter,joy, smiles, hugs, I love you's and forgiveness than by coal and lectures.
They know their behavior is wrong. They know they are screwing up their life with their grades. They don't care about it right now...they are feeling really low. They don't need you telling them what they already know. They need you loving them, being patient with them and then loving them some more.
They don't need to be showered with meaningless gifts. I would personally get them useful stuff. New underwear, pants, shirts, school supplies..maybe a cool movie to watch together Christmas night. But I wouldn't buy gift cards or cool gadgets or video games. Buy them useful essentials.
Have you all ever done family counseling? It is hard to save your kids when they are almost out the door on their own but it is worth a try.
I hope you have a nice Christmas together.