gakkou no kaeri ni, konbini de mizu o kaimashita.

It means on the way back from school or when returning from school.

This expression is very common in Japanese:

• 学校 = school
• 帰り = return, the way back
• 〜の帰り = on the way back from 〜
• 〜の帰りに = on the way back from 〜, at the time of returning from 〜

So 学校の帰りに does not mean school's return in a literal ownership sense. It is a set pattern meaning while coming back from school.

Here, の links two nouns.

• 学校 is a noun
• 帰り is also a noun

So 学校の帰り literally means something like the return from school or the way back from school.

This is a very common use of の: one noun modifies another.

Examples:

• 仕事のあと = after work
• 家の近く = near the house
• 授業の前 = before class

So in this sentence, 学校 tells us what kind of 帰り it is: the return from school.

The に in 帰りに marks the time or occasion when something happens.

So:

• 学校の帰りに = on the way back from school
• The action 買いました happened at that time

You can think of it as similar to:

• when I was on my way back from school
• during my trip back from school

This に often appears after time-related nouns or occasions:

• 昼に = at noon
• 週末に = on the weekend
• 帰りに = on the way back

Because で marks the place where an action happens.

In this sentence, the action is 買いました = bought, and that action happened at the convenience store.

• コンビニで = at the convenience store
• 水を買いました = bought water

So で is correct because buying is an action done in that place.

Compare:

• コンビニで水を買いました = I bought water at the convenience store.
• コンビニに行きました = I went to the convenience store.

Use に for destinations, and で for the location of an action.

を marks the direct object of the verb.

The thing being bought is 水:

• 水を買いました = bought water

So 水 is the thing directly affected by the action buying, which is why it takes を.

More examples:

• パンを食べました = ate bread
• 本を読みました = read a book
• ジュースを飲みました = drank juice

買いました is the polite past affirmative form of 買う.

• Dictionary form: 買う = to buy
• Polite non-past: 買います
• Polite past: 買いました

So 買いました means bought or did buy in polite speech.

A plain-form version would be:

• 買った

So the full sentence in plain style would be:

• 学校の帰りに、コンビニで水を買った。

Because Japanese often leaves out the subject when it is understood from context.

So even though English usually needs I bought water, Japanese often just says:

• 学校の帰りに、コンビニで水を買いました。

The listener usually understands who did it from the situation.

Depending on context, the subject could be:

• I
• he
• she
• they

But if this is a normal personal statement, English will often translate it as I bought water...

Yes, but it is a loanword.

コンビニ comes from the English phrase convenience store. In Japanese, it has been shortened to コンビニ, which is very common and natural.

This kind of shortening happens a lot in Japanese loanwords.

Examples:

• パソコン = personal computer
• スマホ = smartphone
• エアコン = air conditioner

So コンビニ is the normal everyday word Japanese speakers use.

Because Japanese sentences normally place the main verb at the end.

The basic pattern is often:

• time / context
• place
• object
• verb

So here:

• 学校の帰りに = time/context
• コンビニで = place
• 水を = object
• 買いました = verb

That is very natural Japanese word order.

Yes, to some extent. Japanese word order is more flexible than English because the particles show each word’s role.

For example, these can still be understandable:

• コンビニで、学校の帰りに、水を買いました。
• 水を、学校の帰りに、コンビニで買いました。

But the most natural neutral order is usually:

• 学校の帰りに、コンビニで水を買いました。

Even though order can change, the verb usually stays at the end.

Not exactly, though it can often be translated that way.

学校の帰りに specifically suggests:

• on the way back from school
• while returning from school

English after school is sometimes broader. It could just mean any time after classes end.

So Japanese 学校の帰りに has a stronger feeling of during the trip home from school.

Yes, a similar idea could be expressed with 途中で, but the nuance changes slightly.

For example:

• 学校からの帰り道の途中で、コンビニで水を買いました。
• 学校から帰る途中で、コンビニで水を買いました。

These make the in the middle of the trip back idea more explicit.

By contrast, 学校の帰りに is shorter and very natural in everyday speech. It already implies on the way back from school, so it is often the easiest choice.

The comma shows a natural pause.

• 学校の帰りに、コンビニで水を買いました。

It separates the opening time/context phrase from the rest of the sentence. Japanese commas are often used more flexibly than English commas, so this comma is helpful and natural, but punctuation can vary depending on style.