En el comedor de la escuela, mi hermana pequeña come con sus amigas.

Because en shows location, while a or al usually shows movement toward a place.

• En el comedor de la escuela = in the school dining hall / cafeteria
• Va al comedor de la escuela = She goes to the school dining hall

So in your sentence, the person is already there, and the sentence tells us where she eats.

Here, comedor means the dining room, canteen, or school dining hall.

In Spain, el comedor is a very common word for the place in a school where students eat lunch. In a house, el comedor can also mean the dining room.

De la escuela means of the school or more naturally in English, the school’s.

Spanish often uses de where English uses ’s or a noun used like an adjective.

So:

• el comedor de la escuela = the school dining hall
• literally: the dining room of the school

This structure is very common in Spanish.

In Spanish, adjectives often come after the noun, especially when they describe a basic, literal quality.

So:

• mi hermana pequeña = my little sister / my younger sister

With family words, pequeña after the noun often refers naturally to the younger or little sister.

If you say mi pequeña hermana, it sounds more marked or emotional, as if you are emphasizing affection, tenderness, or style, something like my dear little sister.

Because Spanish possessive adjectives like mi, tu, su, nuestro usually replace the article.

So you say:

• mi hermana = my sister
• not la mi hermana

This is different from English only in form, but it is completely normal in Spanish.

Come is the third person singular present indicative of comer = to eat.

Conjugation of comer in the present:

• yo como
• tú comes
• él / ella / usted come
• nosotros comemos
• vosotros coméis
• ellos / ellas / ustedes comen

Since the subject is mi hermana pequeña, which is she, the verb form is come.

Spanish uses the simple present very often where English might use either eats or is eating, depending on context.

So come can mean:

• eats regularly / habitually
• is eating right now, if the context makes that clear

If you want to strongly emphasize that the action is happening right now, you could say:

• Mi hermana pequeña está comiendo con sus amigas.

But the original sentence is perfectly natural.

Yes — sus can mean several things depending on context:

• his
• her
• its
• your (formal singular)
• your (plural, in Spain usually vuestro/vuestra/vuestros/vuestras is also used)
• their

In this sentence, context tells us that sus amigas means her friends.

This is very common in Spanish: the possessive can be ambiguous unless the context makes it clear.

If needed, Spanish can clarify:

• las amigas de ella = her friends
• las amigas de ellos = their friends

Because sus must agree with the noun it describes, not with the owner.

• amigas is plural
• so the possessive must also be plural: sus

Compare:

• su amiga = her friend
• sus amigas = her friends

This is an important point: su/sus agrees in number with the thing possessed, not with the person who owns it.

Because amigas tells us the friends are female.

• amiga = female friend
• amigas = female friends
• amigos = male friends, or a mixed group of males and females

So the sentence specifically says she eats with her girl friends.

Con means with.

So:

• come con sus amigas = she eats with her friends

It shows company here — who she is eating together with.

The comma separates the opening location phrase from the main part of the sentence.

• En el comedor de la escuela, mi hermana pequeña come con sus amigas.

This gives a slight pause and highlights the setting first: In the school dining hall, ...

You could also say:

• Mi hermana pequeña come con sus amigas en el comedor de la escuela.

That version is also natural. The meaning is basically the same, but the focus changes slightly.

Yes. Spanish word order is often more flexible than English, especially with time and place phrases.

For example, all of these are natural:

• En el comedor de la escuela, mi hermana pequeña come con sus amigas.
• Mi hermana pequeña come con sus amigas en el comedor de la escuela.
• Mi hermana pequeña come en el comedor de la escuela con sus amigas.

They all mean roughly the same thing, but the first one puts more attention on where the action happens.

Both exist, but in Spain, colegio is often more common for a primary school, while escuela is also correct and widely understood.

Depending on context:

• colegio often suggests school
• escuela can mean school in general, and it is also used in many set expressions

So the sentence is correct, but in some situations a speaker from Spain might also say:

• En el comedor del colegio, mi hermana pequeña come con sus amigas.

Both are natural Spanish.