The Proportions of Boys and Girls Who Achieved High Grades.

The figure fornumber of boys who took courses in the humanities in boys significantly increased to nearly 45% in 2000, whichere it reached its peak and was overroughly two times more than it was in 1996.

As for girls, however, there hadwas a clear decline, from almost 35% in 1996 to 25% in 2000.

The proportion of boys getting high grades inin the arts also had an upward trend, rising to over 20% in 2000, even though the number of arts had a slight drop over a span of four years.

I can't tell from the graphs whether they represent grades. I also don't see a drop.

The data for the remainingwo of three subjects infor boys illustrateds an opposing trend.

Science and math decreased but it remained the same in Languages.

The percentage of science infor boys experienced a dramatic decrease, while the same subject infor girls had a tiny growth of 1%.

MThe number of boys who took maths fell to approximately 16% for high-achieving boys in 2000, but there was a three-fold surge in the figure for girls to 15% in the same year.

Overall, boys primarily improved their performance in arts and humanities while girls made progress in science and maths from 1996 to 2000.

In the US, we use "math" unlike other English-speaking countries.

The proportion of boys achieving high grades is much higher than girls in all five subjects in 2000, except for the arts and languages.

The figure forproportion of boys achieving a high grade in humanities in boys significantly increased to nearly 45% in 2000, which reached at its peak, and was over two times more than it was in 1996.

Because a paragraph normally means there's a change in subject, "the figure" needs to be clarified (even though it's the title!).

As for girls, however, there hads been a clear decline, from almost 35% in 1996 to 25% in 2000.

The proportion of boys getting high grades in arts also had an upward trend, rising to over 20% in 2000, even though the number of, for girls, arts had a slight drop over a span of four years.

It's not clear what "number" has a slight drop.

The data for the remaining three subjects in boys illustrated anthe opposingte trend.

This isn't actually true, as (you note) languages remained static. So instead: "The data for science and maths illustrated the opposite trend."
"Opposing" is OK, but ambiguous, as it could also mean that the three languages all have different trends.

The percentage of high grades in science infor boys experienced a dramatic decrease, while the same subject infor girls had a tiny growth of 1%.

As you wrote it, it means that "the amount 'science' in boys is decreasing". So you need to say 'high grades'.

MHigh grade achievement in maths fell to approximately 16% for high-achieving boys in 2000, but a had three-fold surge in the figure for girls to 15% in the same year.

It is high grades in maths that is falling, not 'maths', and not specifically for 'high achieving boys'.