I'm sure you could make a case out of it. If you can present your concession card in some appeal process, then you can argue you weren't fare-evading because you were entitled to a concession card whether you were checked upon or not, hence you genuinely made a mistake and should be let off with just a warning.
That being said, surely you must of suspected buying a concession ticket without evidence that you are/were a student, was a bit dodgy in that anyone could claim they were a student, like now in your situation.