As long as you are wiling to work hard, be consistent in your
work, I believe that nothing is impossible in terms of studies.
It's like there is no ugly girl out there, only lazy girls.
It goes the same for studies, there is no stupid students, only
lazy ones.

Both your 2nd and 3rd comments are wrong.
Some girls are born...not so nice to look at, and require some
facial surgery. No matter how hard they exercise, eat healthy food,
they will still scare the daylights out of others. Just remember
that facial surgeries are not cheap.
As for students, just as any good teacher will say, there is no
such thing as stupid students, but there are slower learners and
students who have mental blockages. Some tutors and
parents/guardians I have seen have nastily and cruelly labelled
these students as idiots or useless, and do you know what is the
biggest wrong they could accuse them of? Being lazy. These students
are not lazy, but they are not well motivated, as they are
surrounded by a lot of negative factors. In this cases, the tutors
and teachers have to work VERY hard to show the students that they
have hope, and allow them to give themselves the chance.
I have such a student myself, and I sometimes find myself
committing the mistake or forcing him to work even harder, even
though I know he has a mental blockage problem. His guardian and
relatives often call him stupid or lazy, but I know better
myself.
Trust me, if you are not an experienced or dedicated
tutor/teacher, you won't be able to differentiate these students
from those who are truly lazy.