WATCH: Substitute Teacher Argues with 8th Grader over Thin Blue Line Mask

A middle school student claims a substitute teacher provoked a verbal argument about a Thin Blue Line flag on a mask the boy wore to his class.

The incident took place at Lyman Gilmore Middle School in Grass Valley, California, and was recorded on cellphones by other students Monday, CBS Sacramento reported Wednesday.

Thirteen-year-old Lucas Lillar said he was in his math class when the substitute teacher sat down at a desk behind him and allegedly provoked an argument about the image printed on his mask.

The “Thin Blue Line” looks like the American flag but features a blue stripe to show support for law enforcement officers.

Lillar’s father and stepmother work as members of law enforcement and he said he wears his mask to honor them.

In video footage obtained by the outlet, the substitute teacher apparently compared the flag with the Confederate flag, and at one point stated, “That’s not the American flag that’s the new confederacy flag.”

“I was extremely angry and heartbroken, that this…that he had to deal with it,” Amanda McCallum, Lucas’s mother, recalled.

In a social media post Monday, McCallum detailed the incident and shared a photo of the boy wearing his mask:

In a subsequent post on Thursday, she shared additional video footage and urged others to “teach our children to stand up for what is RIGHT, even when it’s hard”:

McCallum said she contacted school administrators after learning about what happened and was informed by Lyman Gilmore’s principal the substitute teacher would not be able to teach at the campus in the future.

Andrew Withers, who is the Grass Valley School District Superintendent said in an email to CBS the incident was under investigation.

He added, “The California Education Code §44953 allows for the immediate dismissal of a substitute teacher. Our district expects all educators to perform their duties with utmost respect and professionalism.”