By Keith Coffman

DENVER (Reuters) - Students at a Colorado school district may be excited about the upcoming debut of Peyton Manning as the Denver Broncos' new quarterback, but they are banned from wearing the NFL star's number 18 jersey to class.

A dress code imposed by school officials in Greeley, Colorado prohibits students and staff members from wearing clothing that may be associated with gang activity - including the number 18, which could refer to the Los-Angeles-based 18th Street gang.

The ban went virtually unnoticed until this week when third-grader Konnor Vanatta wore his orange Manning replica jersey to school in Greeley, about 50 miles northeast of Denver.

"They told me I couldn't wear 18 anymore because it's a gang number and I had to take it off," he told a Denver television station.

The Broncos open their regular season schedule on Sunday night at home against the Pittsburgh Steelers, a team they defeated in last year's playoffs.

The Greeley-Evans School District said in a statement that the policy was enacted four years ago because street gangs associate themselves with the numerals 13, 14, 18 or their reverses. The only exceptions to the ban are if those numbers are worn in sanctioned team sports.

"Because of the prevalence of some gangs in the Greeley area, we are vigilant about keeping gang behavior out of our schools," the statement said. "By banning these displays in our schools, we have reduced the presence of gang activity, intimidation, recruiting and confrontations."

The district said in order to "send a consistent message" the policy is applied to all students in the district from elementary to high school.

Greeley police spokeswoman Sgt. Susan West said gang members frequently wear certain colors or numbers on their clothing to denote their gang affiliations.

"We are in agreement with the school district on efforts to curtail that activity," she said.

The Broncos signed Manning in March after the perennial All-Pro signal caller was let go by his previous team, the Indianapolis Colts. Manning sat out all last season following neck surgery.

(Editing by Dan Whitcomb and Cynthia Johnston; Editing by Leslie Gevirtz)