It is not clear to me what happened here, but this is what it looks like:
When you say you "marked [your] ticket" it sounds like you are talking about a flexible rail pass (not a ticket), where you fill in the date for each day you use the pass, and ride free that day. You say that the teacher told everyone to fill in the date before you traveled on the 1st, and presumably you did that. You then bought a separate ticket for that day's travel because it was cheaper than using up a day on the rail pass. That can be a smart choice, one that we have often made ourselves while traveling in Switzerland on a flexible pass.
However, once you fill in a date on the railpass it uses up that date and is irrevocable. Even if you had a separate ticket and used that for the trip you made that day, you cannot undo the date you filled in on the pass.
If this is what occurred, it is unfortunate for you, but the train conductor was correct that the date filled in on the pass used up a day, even if you had a separate ticket for that day.