If I believe a student lacks the preparation to pass, what should I say to them? I'm an Advanced Level math teacher in my country. I teach  two courses Pure and Applied. Its duration is 3 years. At the end of three years, there is one exam for the whole three years. Only 1200 students are selected for universities out of 50000.

I told (Privately) two of my students that they are going to fail the August AL exam if they are not going to work hard. 

They have no knowledge of their syllabus. (They joined my class three weeks ago). I myself told them the truth and they stopped the class telling me that I'm a very discouraging teacher. I could have told them "Yes you can," but as a teacher I told them the reality.

Is it better to tell them that "You will get an A" or to tell them the truth?

This course contains 40 lessons and it is a 3 year course from which now only five months are left.
With only three weeks you can already tell they are bound to fail in five months? Yes, that is too discouraging.

There is a middle ground between "you will get an A" and "you will most certainly fail", i.e., 


  If you want to pass, you'll need to strengthen this and that and do a lot of exercises on the material of the class. I know August looks far away, but actually, it will come sooner than it seems, so I suggest you start working on this right away.


By assuring them they are going to fail you have discouraged them from even trying, and possibly planted a predisposition: they know they will fail, you know they will fail, so they will fail (or you will fail them, or they may think you graded them too strictly to fulfill your prophecy).

Your task as a teacher is to help them learn as much as possible, and at the end, assess if they have learnt enough and grade them. I was recently teaching some programming courses, and some of my students were really bad. I knew one of them wouldn't be able to finish the tasks on time, but he took a bunch of tutorials and painstakingly went through them, trying to understand every step of the way. At the end, he didn't know enough to pass, but certainly learned more than if he had just given up at the first try; and he knew that was a possible outcome.