An elementary school principal in Michigan is under investigation for authorizing a field trip last week for 30 black students to meet with an African-American rocket scientist. Students who are not black were excluded from the outing -- a possible violation of a state law that bans racial favoritism in public schools.

"The district is investigating the allegations of violation of the State of Michigan Proposal 2," a spokeswoman for the Ann Arbor, Mich., school district told FoxNews.com. "There was no ill-intent or malice in the principal and teachers planning this field trip," she added.

The principal, Mike Madison, who is black, said the trip was part of an effort to close the achievement gap between black and white students. But some parents whose children were not included say it clearly was illegal.

The controversy began last week when the 30 students, members of an African-American academic support group, were taken to hear the rocket scientist, Alec Gallimore, speak at the University of Michigan, where he is an aerospace engineering professor and propulsion lab director.

The goal of the trip, Madison said, was to close test score gaps and inspire the students to consider careers in the sciences.

But parents of students who were excluded protested, and the children who went on the trip were booed by their classmates when they returned to school.

Earlier this week, Madison tried to quash the controversy by sending a letter home to parents, in which he wrote:

“In hindsight, this field trip could have been approached and arranged in a better way.

"But as I reflect upon the look of excitement, enthusiasm and energy that I saw in these children’s eyes as they stood in the presence of a renowned African American rocket scientist in a very successful position, it gave the kids an opportunity to see this type of achievement is possible for even them.

“It was not a wasted venture," he continued, "for I know one day they might want to aspire to be the first astronaut or scientist standing on the Planet Mars....

“The intent of our field trip was not to segregate or exclude students, as has been reported, but rather to address the societal issues, roadblocks and challenges that our African-American children will face as they pursue a successful academic education here in our community.”

But Madison's explanation only fueled the controversy, and parental complaints turned into allegations that the school had violated Proposal 2, a newly enacted Michigan law that bans racial preference in public schools.

"If it was directed, guided, organized by the school district, they cannot say they are doing a field trip today for blacks only, or for whites only, or for Hispanics only or for Asians only," Leon Drolet, the former chairman of the Michigan Civil Rights Initiative, told the Detroit Free Press.

But district spokeswoman Liz Margolis said that bridging the gap in test scores between white and black students was a serious issue that legitimized the trip. Plus, she said, the field trip was paid for by a private donation.

"We don't feel that it at all violates (Proposal 2), but frankly, as with any group of students, if we identify a group of students that need support, we would be addressing that," Margolis told the Free Press.

"But we also have to have better education for our parents so they know why it's being done," she said.

A parent-teacher meeting to discuss the issue is scheduled for Thursday night.