Is “I’m excused” correct?

Question

I live in France and last week we had to write a dialogue in which I wrote:

But Sir my parent called I am excused

I don't know if it is correct.

We were supposed to write a dialogue where you arrive late to your first day of school and explain to your teacher why was that. In case my sentence is completely incorrect and you're not understanding a thing it's supposed to mean that my parent called the school to say that I will be late for a reason that wasn't the fault of anyone here I was stuck in the subway in between station, so you can't punish me "I'm excused" I have been forgiven for my delay. I don't know if I'm making any sense to any one right now, but it is a very common expression in France, and I figured I could use it.

Answer 1

First of all, you wouldn’t usually say parent even if both parents didn’t call the school.

You would either say

My mum/dad called...

or

My parents called...

Usually, if referring to one parent, you would say which parent it is.

The rest of the sentence is correct apart from the fact that you would add a comma:

But sir my parents called, I’m excused

Excused is quite common to use and is perfect to say what you are trying to say in one word.

Answer 2

There are two sentences here:

My parent called [the school].

And

I'm excused.

Though these are both short sentences, you should not run them together. You should put a full stop (or perhaps a semicolon) between them. Or perhaps it would be better to link the sentences. Looking at the rest of your question, you should learn to use more punctuation, especially to separate the sentences. The lack of clear punctuation makes your question hard to read.

Secondly the phrase "I'm excused" is not very idiomatic in English. It might be understood to mean "I have permission to leave". Possibly it would be understood to mean "I am forgiven (by you)" Neither interpretation seems likely. You can't say that you are forgiven, since it is not your parent's role to forgive you. Only the teacher or the school can forgive you, because it is the school's rules that you have broken by being late. You need to ask to be excused. You can't state that you are excused. And if you are asking for forgiveness, you need to explain why you deserve to be forgiven:

But sir, my train broke down between stations, and that is why I was late. My parents have phoned to explain this. Please, can I be excused?