Don't worry about it.
In the beginning it is always difficult but never say you cannot do it. It's okay to admit you're having a difficult time, but that you will do your best and get better as you go forward in your training.
50 pushups is going easy.
15 good ones? Next time try for 17 good ones. If you do 16, you did one better than previously.
So don't worry.
Your teacher should push you, but you just keep on according to your capabilities and push a little further.
Before you know it, you'll be doing 50, 60, 80 and 100 and beyond.