As a result, the Mount Pleasant Oilers may have to forfeit a few end-of-season victories. The Michigan High School Athletic Association has been notified.
"It's an unfortunate situation," coach Jason McIntyre said Saturday.
The player told the school his name was Javier Jones, but his real name is James Nash, athletic director Jim Conway told The Morning Sun (http://bit.ly/VJz8zc). Nash moved to Mount Pleasant from Lansing.
After a tip, the school investigated, and Nash was expelled Thursday, Conway said.
"There are some official documents that have been altered, so now it's a police matter. He went under an alias, his age was doctored and we weren't aware of altered documents," Conway said.
Under state rules, a student isn't eligible to play sports if he or she is 19 before Sept. 1.
"The MHSAA did indicate that it is unprecedented that a young person has changed their age from older to younger to be able to play. They had not run into this," Conway told the newspaper.
Nash could not be reached for comment Saturday; there was no phone number listed for him.
It's not clear why a 21-year-old wanted to enrol in high school and play football. A message seeking further comment from Conway was not immediately returned.
McIntyre said Nash asked to play football on the first day of school, even though he had missed a few games and grueling summer practices. Nash, known then as Jones, played defensive back and was a starter in the final weeks of the season.
"Did he look older than he was? He looked like any other athletic senior," McIntyre told The Associated Press. "I don't think he had any interceptions. He was not a dominant player."
The Oilers didn't qualify for the playoffs and finished the season with a 4-5 record.
Brandon Isham, 16, said he played basketball with Nash outside of school and didn't know he was 21. He said Nash had talked about trying to get on the school's basketball team.
"He's an excellent athlete," said Isham, who no longer attends Mount Pleasant High School. "He's a good shooter — beyond good."