Colter, however, called the decision a "strong ruling" and predicted it "will be hard to overturn."
Last week's ruling by a regional National Labor Relations Board director in Chicago said Northwestern football players on full scholarships are employees of the university and have the right to form a union and bargain collectively.
While the athletes' effort has generated some support among Democrats, Education Secretary Arne Duncan and the White House have declined to comment on the ruling. Sen. Lamar Alexander, R-Tenn., and Rep. John Kline, R-Minn. — two lawmakers influential on education and labor issues — came out against it.
The university has said it would file a request for the full board in Washington to review the decision. It has until April 9 to do so.
The federal agency does not have jurisdiction over public universities, so the push to unionize athletes has been primarily targeted toward private schools such as Northwestern.
Opponents say giving college athletes employee status and allowing them to unionize could hurt college sports and higher education in numerous ways.