La meva amiga és a casa perquè avui no hi ha escola.

In Catalan, a possessive usually goes together with a definite article:

• la meva amiga = my friend
• el meu llibre = my book
• les nostres cases = our houses

That is different from English, where you normally say just my friend without the.

Because meva has to agree with amiga.

• amiga is feminine singular
• so the possessive is meva: la meva amiga

Compare:

• el meu amic = my male friend
• la meva amiga = my female friend
• les meves amigues = my female friends

So the possessive changes for gender and number.

Yes, you can also hear està a casa, but és a casa is very normal Catalan.

Catalan does not use ser and estar in exactly the same way as Spanish or English. With location, Catalan often uses ser where English simply says is:

• És a casa = She is at home

Està a casa is also possible, often with a stronger sense of someone's current or temporary presence there. For a learner, the safest thing is to remember that és a casa is correct and natural.

A casa is a fixed expression meaning at home or home.

So:

• és a casa = she is at home

You do not normally say a la casa for this meaning.
But with many other places, Catalan does use an article:

• a l'escola = at school / to school
• a l'oficina = at the office

If you want to specify whose home it is, you can add that:

• a casa seva = at her home
• a casa dels avis = at the grandparents' house

Here perquè means because.

So the structure is:

• és a casa perquè... = she is at home because...

It introduces the reason.

Because perquè meaning because is written as one word in Catalan.

It is different from per què, which means why or for what reason.

Compare:

• No hi ha escola perquè és festa. = There is no school because it is a holiday.
• Per què no hi ha escola? = Why is there no school?

So this spelling difference is important.

Hi ha means there is or there are.

It is the standard Catalan existential expression:

• Hi ha un llibre. = There is a book.
• Hi ha dues cadires. = There are two chairs.

In your sentence:

• no hi ha escola = there is no school / there isn't school today

It is best to learn hi ha as a whole chunk.

The hi is part of the fixed expression haver-hi, which is used to mean there is / there are.

Historically, hi is a little locative element, roughly like there, but in modern Catalan learners usually just treat hi ha as one unit meaning there is/are.

So you normally do not translate hi separately in this phrase.

Because escola here means school/classes in general, not a specific school building.

So:

• avui no hi ha escola = there is no school today / there are no classes today

This is similar to English, where no school today usually means no classes today, not there is no school building.

If you were talking about a specific school, you would normally use the article in a different kind of sentence, for example:

• L'escola és tancada avui. = The school is closed today.

Avui means today, and Catalan is fairly flexible about where time expressions go.

In this sentence, avui comes before no hi ha escola to make the time reference clear right away:

• perquè avui no hi ha escola

That is very natural. You could also hear:

• perquè no hi ha escola avui

but the version with avui earlier often sounds more neutral and natural.

És is the third-person singular form of ser:

• ella és = she is

The accent marks the stress and also helps distinguish it from es, which is a different word in Catalan, often a reflexive pronoun.

So:

• és = is
• es = himself/herself/itself, or part of a pronominal verb

The accent is important in writing.