The district bans 13, 14 and 18 and their reverse, 31, 41 and 81. Dreessen wears 81.
The rule is three years old but made headlines this week after a third-grader wearing Peyton Manning's No. 18 was sent home to change.
Dreessen said Thursday the ban doesn't bother him if it prevents trouble.
Eighteen is in the name of the 18th Street gang, while 13 and 14 are associated with other gangs but not used in their names.