There is a modern normative convention but there was never an official standard, and the Roman's usage actually had a lot of variation. Your teacher may have been right that some Romans actually wrote IIMM, but he certainly wasn't right to claim you were wrong.
My primaryschool maths teacher taught us roman numerals and one of the tasks we got was to write out the current year in roman numerals.
I came up with MCMXCVIII … to which he smugly replied that it's wrong and the romans were a lot more clever and it's just IIMM (take 2 off 2000).
Years later I learned that he was quite wrong about that and my answer is in fact the only correct one.
TFW your school insists you learn something utterly pointless and then the teacher teaches it completely wrong.
IIMM just looks so silly. If that were allowed, then why would the Super Bowl roman numerals be so long in the 1900s?
(I don't even watch football.)
There is a modern normative convention but there was never an official standard, and the Roman's usage actually had a lot of variation. Your teacher may have been right that some Romans actually wrote IIMM, but he certainly wasn't right to claim you were wrong.