@Unnamed,
You don't mean first grade in America. For us, that's full of 6 year old children. You mean freshman year, also known as ninth grade, which is generally 14 year olds.
Be that as it may, if you cannot tell this girl how old you are without there being dire consequences, then she is far from perfect. Discussing your age is something that should be totally consequence-free. I suspect it's more likely that you're just terrified that if you contradict her in any way, you'll blow your chances.
This is what boys have been thinking about girls since people were wearing animal pelts and living in caves. It was rarely true then, and it's rarely true now.
Have a little confidence in yourself, and in this girl.
Be nice, be polite. And for God's sake, ask for her contact information.