grammar - How does までに work with 少しずつ

日本の小学校は、２０２０年度までに英語の授業を少しずつ増やすことになっています。

Japanese primary schools are expected to increase English classes gradually by 2020.

I'm confused about the combination of までに (by) with 少しずつ (little by little) . The direct English translation makes no sense. You can't do something 'little by little' by a certain time. I think the only way I could understand this is "they have until 2020 to start increasing the number of English lessons'.

You can do something 'little by little' until a certain time.

You could also say something like 'expected to increase classes little by little until they reach two hours a week by 2020'

So I think my question is why までに rather than just まで? What does this sentence actually mean?

Answer

It's interesting to see how まで（に）seems to confuse learners of Japanese as much as the distinction betwenn "by" and "until" confuses the Japanese when they learn English.

To expand slighly on user4092's explanation, までに simply expresses a period between now and a defined future point in time. In this sentence, the 少しずつ simply describes how things will be done during that period.

The point of view adopted is that of the end result: "by the time the 2020 school year comes around, the number of English classes in primary schools will have increased compared to now. The process of increasing them will proceed little by little between now and then."

Addendum: the above explanation is simply an illustration of the viewpoint adopted when までに is used, not a proper translation. Your reading of 増やす as "will increase" is correct. The ことになっています here simply expresses that everything preceding it is fact. Although more awkward in terms of English, a translation closer to the structure of the Japanese would be something like:

It has been decided that primary schools in Japan are going to increase English classes gradually between now and the 2020 school year.

where the part in italics corresponds to ことになっています, and the part in bold is までに.