Six Arizona high school students have come under fire after allegedly posing in t-shirts spelling out a racial slur.

The Desert Vista students had been part of a senior picture day photo, wearing t-shirts with a letter of the slogan 'BEST*YOU'VE*EVER*SEEN*CLASS*OF*2016', ABC15 reported, but six of them were pictured having rearranged the order to spell out 'N****R'.

The photo was originally thought to have been posted to the app Snapchat, which immediately deletes photographs, but it has since been widely shared on social media, causing outrage.

Tempe Union Superintendent Dr Kenneth R. Baca said in a statement: "The event that took place today at Desert Vista will not be tolerated.

"It is unacceptable for any racial slur to be used regardless of intent. We need to understand that words matter. I can assure you the appropriate disciplinary action will be taken."

Since the image emerged on 22 January, more than 36,000 people have signed a petition calling for the students to be permanently expelled from Desert Vista high school, in Phoenix, and the principal to be dismissed.

The petition, on, was started by Bri Handy, from Phoenix, and read: ""Arizona has been stricken with another blatant expression of racism and ignorance.

"Six students at Desert Vista High School were a part of a larger group of students that adorned shirts with letters to spell out a celebratory slogan for the school year.

"The six students in question pulled ranks to spell out N****R on their own, smiling gleefully to pose for an instagram picture.

"Their punishment was 5-days suspension. This hurtful use of a racial slur is a complete disregard for the dignity of the black community in Arizona and across the nation and the punishment does not fit the total ignorance and cruelty of the crime."

The picture was also widely shared on social networking site Twitter, where many students at Desert Vista commented on what had happened.

Several users were embarrassed that their school had been so widely publicised following a racist incident, with one commenting: "Thumbs down to the Desert Vista girls. I know you've all seen the pictures."

The school issued a letter to parents following the incident, stating that the students involved would be punished and that security was being stepped up at the school 'as a precaution'.

It read: "Dear Desert Vista Families, We want to let you know we share your outrage and disappointment over the actions of a handful of our students today.

"We want to make it crystal clear those actions are unacceptable and will not be tolerated at Desert Vista. Those actions do not represent who we are as a campus.

"We can assure you we will discipline these students in accordance with district policy and our student handbook.

"While we don't discuss individual discipline, we can tell you that in addition, the obvious need for sensitivity training will be addressed.

"As a precaution, Phoenix Police will be increasing security at and around Desert Vista."

The students are believed to have been suspended for five days, according to classmates on social networking sites, although the school mentioned that it would not comment on individual disciplinary cases.