We all mistakes. That is absolutely normal. That is the way we learn. Speaking about failing and making mistakes at school, teachers should not be so concentrated on making sure that everything that their students do is perfect.
To begin with, it is not the way in which it happens in life and sooner or later students are going to learn that. When they figure out that at school, it will be easier for them to adapt to the harsh realities of life. What is more, failing and making mistakes is huge part of the studying process. It is the way in which a student discovers what way is better in order to reach their goal and what way they are going to avoid the next time they face some sort of dilemma.
Another important aspect to mention is that making mistakes motivates a student to repeat everything they have been doing once again, yet to do it better this time. It is the natural way of getting better at something.