And if he was breaking the rules, then speaker and party colleague Mitchel McLaughlin played the tough headmaster.
He could not deliver 100 lines but he delivered a dressing down for not dressing up and breaking the assembly's dress code.
"You are well aware that it (the code) applies to you," he told Mr Flanagan.
He would, he said, be "rigorous" in making sure all MLAs  followed suit. But what suit?
The scolding sparked a lively debate on social media about what was the right way to dress for the chamber.
A Northern Ireland Assembly spokeswoman has confirmed that there "is no exact dress code" adding: "The speaker expects members to wear 'business attire'".
"In a ruling by the speaker on 19 June 2007, the speaker addressed the issue of dress code in the chamber," she said.
"In a leaflet on rules of behaviour and courtesies in the House, distributed to all members in January 2014, the ruling was reiterated as follows: 'While there is no exact dress code, the way in which you dress should demonstrate respect for the House'."
Some social media users described the ruling as "archaic" and compared the rules in Northern Ireland with the laid-back look of independent Irish TD Mick Wallace - sporting a green t-shirt in the Irish Parliament.
The dress code in the Irish Parliament (Dail) is not as strict.
Since the 1980s, Independent TD Tony Gregory wore a jacket, but never a tie, and jackets are not worn by mainly left-wing TDs such as Mr Wallace and Richard Boyd-Barrett.
According to the Code of Parliamentary Standards in the Irish Parliament, "Members should dress in a manner that reflects the dignity of the House".
In previous years, proposals for a more formal code from some parties has been met with stiff resistance by others.
The parliamentary dress code in the House of Commons is similar to that of the Northern Ireland Assembly. It has been broken too.
In October 2014, Labour MP Harriet Harman wore a t-shirt with the slogan "This is what a feminist looks like" to Prime Minister's Questions.
Speaking at the time, a House of Commons spokesman said: "If a member's dress is not in line with the conventions of the House and it is brought to the chair's attention, he acts accordingly."
Limited rules on members' dress are set out in Erskine May, the official parliamentary rule book.
It says MPs should not wear military insignia or uniforms in the Commons and the custom is "for gentlemen members to wear jackets and ties".
Women are expected to dress in "business-like" attire.
In 2010, one Labour MP complained that some women MPs had been "rocking up in a mixture of denim and knee length boots".
Following Phil Flanagan's scolding, opinion on the parliamentary dress code on social media was divided with some saying they would expect smart clothing to be worn, although one Twitter user did ask, "Does a tie really matter?"
No tie and the top two buttons undone - Sinn Fein's Phil Flanagan had a look of the rebellious schoolboy about him in the Northern Ireland Assembly earlier this week.