Meg and Dean just happened to be sitting at the same table in study hall that day.
Meg had actually been planning to talk to Dean and drop some hints about Castiel, but they'd been sitting there for fifteen entire minutes and neither of them said a word, much less to each other.
Finally, Dean looked up from his book and said, "Hey, I know you…." his voice trailed off as he tried to remember where he'd seen her, "You and Castiel are roommates, right? I've seen you with him before."
Meg was completely taken aback. "You know Cas?" she asked in awe. "Not really." Dean sighed in response. "You said you've seen me with him before. Was that just a crazy coincidence or were you watching him?" she smiled. When Dean sighed and picked up his History book again, she got her answer.
"Oh my god, you were watching him!" Meg practically yelled. Dean grabbed her arm quickly, pulling her in close. "Be quiet!" he whispered, "Okay, fine, you're right. I was. Please, don't tell anyone. Especially Cas."
He paused. "Come with me. Let's talk in private."