One solution is to create a custom SQL data connection to first calculated the avg NOTA for each student as below:
select NOMBRES, avg(NOTA) as avg_nota from YOUR_TABLE group by NOMBRES
Then you can create a histogram for avg_nota, either with Show Me or manually.
Here is a link to an example based on your original
The SQL above weighs each score equally, which is fine if each course has exactly the same number of grades. But if the number of records varies between courses, you should adjust the approach to make sure each course is weighted the same (e.g. so that a course with 10 small tests does not get weighted twice as much as a course with 5 larger tests). The solution in that case, might involve repeating the above step in a nested subquery or view grouping by both NOMBRE and CURSO. Still this simple approach should give you the basic idea.
The solution above works but I think there ought to be a way to get the same effect using table calculations without resorting to custom SQL