I don't think intelligent students are intelligent forever and so the dull minds are. Let's take example of the Great Poet Kalidas. He was very foolish in the beginning period. But he became genius later. Even the great scientist Sir Isaac Newton is believed to be absent minded. There are many examples in the world which tell you by shouting that dull minds are not dull forever.
If you are a teacher and you believe A is brilliant in your subject and you treat like that with him, then he will be proved to be brilliant for you. Similarly if you think B is dull, he will remain poor in your subject even though he is one of the good students of the class. This is the reason the position of the students change when they migrate from one place to another.
What is the psychological cause behind it? When you treat anyone as a brilliant, his subconscious mind works accordingly and he will remain really brilliant. Oppositely, if you treat anyone as a dull man, he will remain dull at least in front of you.
Kalidas was so foolish that he used to cut the branch of tree by sitting on it, later he believed to have blessed by Goddess Kali to be genius. His strong belief made him really brilliant. So if anyone is to be sharp with mind, first he should confirm his own mind that he is genius.
Discovered by +Don Prince