(Sorry.  The Italki character limit means I must add my comments separately.)

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[1] It's somewhat redundant to refer to a "writing article". A better title would be "A Writing Exercise".

[2] The word 'respectively' follows a list of items only when it refers back to the order of items in an earlier list. For example:

"The charts show the proportion of teachers' salaries as 40%, 50%, and 45% in the years 1981, 1991, and 2001 respectively."

[3] The phrase "in these three years" means a span of three consecutive years (such as 1981 to 1983). To mean three non-consecutive years, you might write "in these three sample years".

[4] The phrase "more and more" is very informal. For example, "His wife has become more and more loving over the years." It is too informal for describing a trend shown in charts.

[5] Starting a sentence with 'but' or 'yet' is available when starting a new sentence on the same subject as the previous sentence. However, the subject is changing from insurance to resources. (This is not a rule. This is a guideline that I have made up to explain why the word 'but' seemed inappropriate here.)

[6] It is ambiguous whether "school resources and equipment" means a single expense or two separate expenses. (I know the chart makes it clear, however your essay must stand alone.) That is why I have suggested writing "on resources and on equipment", to make it clear that you mean two separate expenses.

[7] I don't believe that three fluctuating figures represent a "trend".

[8] We use the word 'remainders' to refer to a very small collection, perhaps as much as 5%. A proportion as high as 25% should not be referred to as an unspecified 'remaining' proportion.

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It seems that I have made a lot of corrections. However, your writing is very, very good.

A Writing Article [1]

The three pie charts show the money spent by one school in the UK in 1981, 1991 and 2001 respectively [2]. We can see that, in these three sample [3] years, the money spent on teachers’ salaries varies varied between 40% and 50% of the total money, which is was the biggest proportion of the annual spending. However, the proportion of cost of spent on salaries for other workers decreases, from 28% in 1981 to 9% in 2001.

From the charts, insurance seems to be have been more and more increasingly [4] important in this the school in these three years over this 20-year period. The proportion of school spending on it increases increased from 2% in 1981 to 8% in 2001. But [5] The percentage of annual spending on school resources and on [6] equipment fluctuates. For example, 15% of the total money was spent on resources (such as books) in 1981. This percentage is was 20% in 1991, but 9% in 2001. While, Yet the proportion of cost spent on school furniture and equipment has had the opposite trend [7], with 15%, 5% and 23% in 1981, 1991 and 2001 respectively.

As a result, we can see that in the particular UK school in these three years over the sampled years, most of the annual spending is was on salaries for teachers and other workers. The spending on insurance occupies bigger and bigger an increasing proportion, although it is was less than 10% in all of those years. The proportion of spending on remainders [8] keeps stayed between 25% to 30%.