Like if she had showed interest before, but slowed down because you were too nervous to approach her. So you saw her in school, and if you smiled at her, and she smiled back, would it be just friendly or would she want you to approach?
It depends on the girl. If a guy smiles at me I always smile back. If I see a guy staring at me in class I make eye contact and then smile. Some girls if they are flirts they are impossible to tell but some of the more shy girls if they smile they at least think of you as nice or a friend.