valedictorian

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The student with the highest academic rank in a class who delivers the valedictory at graduation.

from The Century Dictionary.

• noun In American colleges and some academies and high schools, the student who pronounces the valedictory oration at the annual commencement or graduating exorcises of his class: usually chosen as the scholar bearing the highest rank in the graduating class, as the best representative, for various reasons, of the whole class, or as otherwise worthy of special distinction.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who pronounces a valedictory address; especially, in American colleges, the student who pronounces the valedictory of the graduating class at the annual commencement, usually the student who ranks first in scholarship.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun US, Canada, properly The individual in a graduating class who delivers the farewell or valedictory address, usually the person who graduates with the highest grades.
• noun US, Canada, colloquial The individual in a graduating class who graduates with the highest grades.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the student with the best grades who usually delivers the valedictory address at commencement