Although Edward stares at Bella in the cafeteria, her smell doesn't really hit him until she walks into their science classroom.
Edward immediately tenses up and slams his hand over his nose and mouth when her scent is carried over by a fan.
It's an amusing scene to watch without knowing why Edward is reacting like this, especially when Bella sits next to him and smells herself.
Notably, "Midnight Sun" — "Twilight" author Stephanie Meyer's book from Edward's point of view — revealed that he was struggling not to kill Bella and their classmates when she walked in.