But laboratory tests confirmed recent cannabis consumption, and that weight loss could not account for the positive results.
McMillan finally admitted that he had smoked two cannabis cigarettes.
The disciplinary tribunal ruled that what McMillan did amounted to serious misconduct.
It was also revealed that a mandatory report filed with the council in 2013 had raised concerns around the teacher's drug-related impairment in the classroom.
That investigation resulted in McMillan entering a rehabilitative agreement, which included providing negative drug tests every three months.
"The tribunal noted that while smoking cannabis is not the most serious example of illegal drug use, it did occur while he was subject to his previous agreement with the council," the decision said.
The tribunal censured the teacher and put several conditions on his practising certificate, including having to provide any prospective employer with a copy of the decision, and to undergo random urine tests.
McMillan also has to attend drug counselling and complete a drug counselling assessment.