Santana thinks that Brittany's not graduating, and she knows how upset Brittany was. That's why, when she gets up on stage, she finds Brittany in the crowd straight away. She waves, blows kisses, even mouths 'I love you' (because that's who she is now and she's not afraid of what people will think any more).
What she doesn't know is that Brittany has been taking night classes ever since she found out. That the reason she couldn't go to Santana's house last week is because she had been re-taking one of her exams (because she really just wanted to make Santana proud because if she couldn't do that then what's the point of doing anything?)
Which is why when Principal Figgins calls out for "Pierce, Brittany", Santana grin is so big, she thinks her face is going to split in to two. Which is why when Brittany recieves her diploma, Santana's cheering the loudest. Which is why, when Brittany reaches her, she takes Brittany in her arms and kisses her (because, really, they're leaving both this stupid place anyway, so who cares if anyone's offended?).
And while everybody else's names are being called, Santana whispers, "How? Why?" Because, really, Brittany hadn't seemed to upset by the fact that she wasn't graduating. There's a long time before she answers, "Because I love you." (and they both know that's the truest thing she's ever said, and Santana's choked "I love you too" hers).
And when they're back at Santana's house, cuddled in bed, Brittany blurts out, "I got into Louisville. I mean, it was tough, they nearly didn't take me because I failed the first time round but when I told them about my history with cheerleading too and that I graduated after my tests they said y-" Santana cuts off her rambling with a kiss. "That's amazing, Brittany." There's a pause, and she can feel her heart rate quickening. "Did I tell you my parents bought me a house in Louisville? Well, they did, and it's pretty big and, well... It's big enough for two people, if you know what I mean." (and Santana swears to this day that waiting for Brittany's answer was the scariest moment of her life).
So when Santana puts the key in the door to her, no, their new house, she walks in hand in hand with Brittany and takes a deep breath and kisses her girlfriend and wishes she could live in that moment forever (but then she realises that this is her forever, and then she feels like the happiest girl alive.)