Well gurl, since I'm sitting next to you in class I may as well answer, you should be straightforward with him... Its best not to beat around the bush. Just explain what happened. I mean you're about to graduate so its not THAT big of a deal. But you should get a pregnancy test from the dollar store or something... If you need one I can buy one for you :3
You tell him you want to talk to him privately....then you start telling him and asking him about when you where a baby....them after he's calm you tell him you may be having a baby.... GOOD LUCK!!
Sit him down and tell him there is something important you need to tell him. Hope this helps x