I totally believe you that you have this memory!
But now take a step back and consider that each of those teachers you vividly remember saying the calculator phrase to you, must have had a mobile phone in their pockets, which at the time had exactly three apps presented prominently on the main screen: a calendar, an alarm clock and a calculator.
So why would they have said such a thing to you?
And still, you have that totally realistic and believable memory of it.
Like I also have the vivid, colourful, and absolutely undeniable realistic memory of the "Fruit of the Loom" logo having featured a cornucopia when we were wearing this brand in school.
I think the calculator remark memory being a Mandela-Effect artifact is not a totally unreasonable thing to suspect...