+++Paddington Bear

Thanks, Uly! :)

Mannnnn !!! Why do RUSSIANS write everything sooo short and in abbreviations....if they have the normal forms of them to write ....

It's worth saying that уд(овлетворительно) = 2 mark on Russian 1- 5 grading system where 5 is the top mark.
5=отлично
4=хорошо
3=удовлетворительн
2=неудовлетворительно
1 is used only in exceptional cases

I guess this answers the last question

отл
хор
уд
неуд

You may ask why teachers use these instead of numbers/digits. I am not sure, but most likely because numbers are much easier to forge, e.g. 3--->5, 1-->4, 2-->3

In the English grading system, that would be UNSATISFACTORY

In light of this, I'd say that "Unacceptable behaviour! " isn't the most adequate translation.
It's more like "Behaviour - the lowest mark"

That’s what I meant - by analogy, the lowest mark is “unsatisfactory (behavior)”

unacceptable sounds more like a personal reproach

I agree, but in a list of subjects with marks I'd expect the first column to be for subjects and the second - for marks - the same as in the original ("повед. - неуд.", а не "неуд. повед.")

exactly

Well, in this case doesn't your translation need correction?

✅