Contextual Situation:Australian Employment Contract Law

1. John is a student in his final semester, he then applied for a job and during interview highlighted that every friday he needed to leave 3 hours early to attend class or else who would not accept any job offer. Interviewing supervisor agreed, contract needed to be signed by the CEO but was away.
2. John was offered the job, but was yet to be handed the contract as the CEO was not back, nonetheless, he started work and performed duties as per the verbal agreement, leaving 23 hours early. after several weeks, the contract was handed over to him but didn't get around to sign, and few months later the supervisor denied John from leaving early on friday. John claimed it was a term of the contract. Incidentally, remembered he had not signed the contract, and when he went to check the contract, that clause about leaving 3 hours early on friday was included.

Question:

Did a valid contract came into effect, and if so can John enforce the omitted clause to be a term of the contract?