When she questioned the appropriateness of the conversation - telling him "this is f***ing weird" - he told her she could "delete me if you are chicken", and went on to say he was trusting her not to tell anyone.
At one point he said: "Don't tell anyone we are friends on here ok. Shhhhhhh."
The principal of the college complained to the Teachers' Council and the police, after a teacher at the school relayed a conversation between herself and some female students.
"The girls were concerned about the tenor of the [Facebook] conversation," the teacher said.
The principal was concerned the Facebook exchange could be a precursor to grooming, so contacted the police, who interviewed Mr Duffy.
There were also allegations about comments made in the classroom, and Mr Duffy using "inappropriate hand signals", and making comments about how a student looked in her PE gear.
However, the Teachers' Council disciplinary tribunal said the only evidence of this was in hearsay statements made to the principal.
During his police interview, Mr Duffy denied he had any "intentions", and said he was simply responding to a friend request and "obviously it's backfired on me. The joke that they were playing".
He denied any conversations or acceptance of friend requests prior to this incident.
Police determined the Facebook conversation did not meet the criminal threshold for grooming, and did not lay charges against Mr Duffy.
In his exchanges with the Teachers' Council, Mr Duffy maintained he was the victim of a prank by students, claiming he had been "set up by a couple of girls thinking it would be funny", and that he had been forced to lay a police complaint against the girl involved "because she got hold of my cell number and was prank calling me".
However, he acknowledged to the tribunal that his actions were professionally inappropriate and unprofessional.
He had no intention of going back to teaching, and was gutted by how it had all turned out, he said.
In it published decision, the council said: "The respondent now agrees that his actions were unprofessional, but still tends to minimise them on the basis the students had led him into it and were playing a joke on him."
It went on to say: "The behaviour was at the least extremely foolish, it could be regarded as having a sinister dimension, and it certainly demonstrates a failure to appreciate the nature of appropriate professional boundaries in a way that any reasonable member of the public would find significantly concerning."
Even if the students had been trying to draw him into an embarrassing exchange, the council said, which there was no evidence of, "a teacher should know better".
"Any teacher in 2014 ought reasonably to know that social media is a communication medium which must be used with considerable caution with regard to communications with students."
It added: "To use it for a private one-on-one conversation with a student with the sort of content that was in this Facebook exchange will always be completely inappropriate."
Mr Duffy was censured, and ordered to inform any future teaching employer of the decision.
Transcript of Facebook conversation with 15-year-old student:
Leon Duffy: Hi
Student: Hey man!!!!!
LD: At school?
S: yaaa are you?
LD: good girl. Ha ha. Nah not today. Cruising at home. Why friend request
S: hahahah, lucky! Wtf I want to be at home
LD: Nah. School is cool. Ha ha. Your pics are boring by the way. He he
S: hahaha no way, school shit as
LD: Ha ha. I was super naughty at school.
S: Hahaha why are they boring. What did you do that was naughty? No good ay
LD: I can't tell you, not a good role model. Ha ha
S: ahahah aw algood
LD: What year are you?
S: 10 haha
LD: Oh shit! Really? How old?
S: hahah ya, im 15
LD: WTF? I thought you were older. Ummmmmm...shall we stop messaging now so I don't get in trouble? Hmmmmmmmmmm. What classes have I taught you in?
S: hahahahahahaha I thought you knew me??? and science
LD: I do. I just don't know what classes I taught you in. Why you friend request me. Be honest, promise I won't tell anyone as long as you don't?
S: Hahahhaha me and [redacted] kinda just added you as a joke and then you messaged me, so idk, it was a joke so algoods aye
LD: haha. Sweet as then. Hey...don't tell anyone we are friends on here ok. Shhhhhh
S: yah haha, ahahaha yea I wont
LD: OK cool. I won't delete you then. Ha ha. Hey we can be bad ass and message each other in class next time I have you. He he
S: your like double my age wtf hahahaha
LD: Ha ha. Nah... Your half mine. Ha ha. I win.
S: Hahaha Naaa your like 40
LD: Ha ha shuuuushhhhh. I was being nice when I said I thought you looked older than 15. Ha. Your probably only 12 or something. Ha ha
S: 12? Im year ten hahaha how could I be 12
LD: Ha ha. Really intelligent. Ha ha. And how could I be 40? I'm way too cute. Ha ha
S: I feel like this is f***ing weird
LD: aha. you requested me remember. Delete me if you a chicken. Ha ha
S: your like risking your job, don't you have a girlfriend...?
LD: Why am I risking my job? You told me you wouldn't tell anyone so I'm trusting you. No girlfriend. You have a boyfriend? Ok maybe I can't trust you?
