What is the percentage of a Teenage "Engagement" working?

yeah so i no a guy and a girl Girl (15) Guy (17) both in high school both didn't make it to the next grade because they didn't make enough credits these two have been on off dating since him and i broke it off in march 2010 its every couple months break up and reunite with these to and the last time they broke it off she told him that she didn't love him and that they were done they became friends with benefits and then started dating again btw he kinda moved himself into there apt. and asked her to marry him without a ring and now there "engaged" but see he did the same thing with me purposed without a ring didn't go down on one knee just asked and got a yes....every adult i talked to said its childs play and they won't last even if they did tie the knot...what do you think