Impromptu Performance

I have been getting used to teaching English to students in many generations lately.
However, sometimes I am offered to be a substitute teacher. While I am teaching those classes, I feel like that " I am wrestling with other wrestler's loincloth(Fundoshi!)"
I mean, it is REALLY uncomfortable to me...

Yesterday, I was asked to teach a high school student's grammar class. It was a private lesson on every Thursday by our head teacher, Toshi. I overlapped with Toshi quickly, and he said, "Homework is this page, and tomorrow, please teach these two sections." I prepared for the lesson this morning, and I had the class this afternoon.
However... The homework parts and today's parts were totally different!!
The student, Miki, said, "Today, we are supposed to do this section!!"
I said, "OK, that's fine" trying not to show my shock.
I did "impromptu performance" through the class. I guess it was a poor lesson...
I might have taught her wrong answers...

Why did it happen? Full time teachers in our school are very busy & very experienced teachers. So, sometimes they don't understand exactly which part they did in the last lessons, especially, seasonal special private lessons like Miki.
However, it doesn't matter for them. They can do "impromptu performances" anytime because they are really PROFESSIONAL English teachers!

But I can't do the same thing for now... I'm embarrassed to think back about today's substitute lesson.
However, I should do these lessons as long as I am offered. And also, I can't grow if I teach just "comfortable lessons."

After the lesson, Toshi said to me... "Thank you VERY much, Keiko sensei! You never refuse my offers, so I appreciate it!!"
What?? CAN I refuse these?? I didn't know that...