[lynguist]

They’re length markers. There’s also the rarely used ˘ to show that a vowel be read short instead of long.

At least in my gymnasium in Switzerland we had the length markers for all the words, from the very beginning, and in all texts we read and all grammar forms we learned.

[toyg]

Seems like a modern addition. I've seen quite a few Roman inscriptions in my life, and I don't remember ever noticing any such marker.

[lynguist]

No, not in inscriptions! (Even though inscriptions did in fact use length markers, there’s the superlong I for example.) When we transcribe actual Latin text we make some changes.

The actual inscriptions use heavy abbreviations, which we resolve in our text. And then we also disambiguate V into v and u. And as a bonus we often add the length markers.

So we do inscription -> cleaned Latin text.

And the cleaned text is given to students.

[saaaaaam]

Interesting. We were just expected to learn it or work it out!