How should I respond to this postcard my teacher mailed me?

I am a high school sophomore and my teacher mailed me a postcard on official school stationery. It said "BRAG" down the side, and "Bulldogs Really Are Great" on top. On the lines she wrote, "Sarah- Great job in Gen Chem this year. Keep it up for one more quarter. :) Mrs. Smith." My dad thought something was wrong because in the 5 years I've been at that school (I was there for middle school), I've never received a "BRAG" card despite all A's. It seemed really random. No one else has ever mentioned getting a card either. My question is, what should I say to her the next time we meet? Something like, "I got your card, thanks"?