I remember it as being similar to BIG with Tom Hanks, in that it's a boy that gets to be a grown up for a while. This was of course around the body-switching era. He falls in love with a girl(blond). What sticks in my mind was the ending, where he's a kid again, back in his classroom, and this little blond girl comes walking in and takes a seat, and you realize it his girlfriend and she's become a kid to be with him. Any takers?