Here I go again with more advice... Since I've already given some today, I might as well not stop now! Take this from a slightly older, wiser person who has been through high school and had a couple years to look back and understand how things work.
1. If you REALLY don't want anyone to know a secret, don't tell a single person; not even your BEST friend. As soon as you tell someone else your secret, you can be sure it won't be a secret long. If you really need to tell someone something, tell your mom.
2. Poohbear, the guy that said that to you is probably interested in you too. If he didn't like you, he would probably avoid you once he found that out. Thats what I would have done at least... Since he came and said something to you, try saying something back to him.
Best of luck to you and all the rest of the shy girls.
~CW