اختي بالمدرسة.

A common Levantine pronunciation is ekhti bel-madrase.

• اختي → ekhti
• بالمدرسة → bel-madrase or bil-madrase

A few sounds to notice:

• خ is the throaty kh sound, like in Scottish loch or German Bach
• ة at the end of مدرسة is usually pronounced -e in Levantine, not -a

You may hear small regional differences, but ekhti bel-madrase is a very useful way to say it.

The -ي is the possessive suffix meaning my.

So:

• أخت / اخت = sister
• أختي / اختي = my sister

In Arabic, possession is often shown by adding a suffix directly to the noun, instead of using a separate word like my.

Because اختي is already definite.

When you add a possessive suffix like -ي (my), the noun automatically becomes definite:

• أخت = a sister / sister
• الأخت = the sister
• أختي = my sister

So ال is not used with it. You would not say الأختي.

In Arabic, especially in the present tense, sentences like this usually do not use a word for is / am / are.

So instead of saying:

• My sister is at school

Arabic simply says:

• My sister at school

This is completely normal. The present-tense to be is usually omitted in both Levantine and Standard Arabic.

Here بـ means in or at.

So:

• بالمدرسة = at the school / in the school

In natural English, this sentence is often translated as My sister is at school.

In Levantine, بـ is extremely common for location, so you will hear it a lot.

Because بالمدرسة includes the definite article ال (the).

It is made of:

• بـ = in / at
• المدرسة = the school

Together:

• ب + المدرسة = بالمدرسة

So بالمدرسة means at the school / at school.

If you said بمدرسة without ال, that would mean something more like in a school or at a school, which is less specific.

The more careful spelling is أختي.

But in informal writing, many people write اختي and leave out the hamza sign. This is very common in texting and casual Arabic writing.

So:

• أختي = more standard spelling
• اختي = very common informal spelling

Both are understood.

Because in Levantine Arabic, the ending ـة is usually pronounced -e when the word is said by itself or at the end of a phrase.

So:

• مدرسة → madrase

In Standard Arabic, the same word is pronounced more like:

• madrasa(h)

This is one of the very noticeable sound differences between Levantine and Standard Arabic.

Usually, in a sentence like this, it means the person is at school as a location.

So اختي بالمدرسة most naturally means:

• My sister is at school

Depending on context, English might also say in school, but the Arabic here is mainly describing where she is.

No. You normally do not need هي here.

اختي already tells you the subject is my sister, and Arabic does not need a present-tense is in this kind of sentence.

So the natural sentence is simply:

• اختي بالمدرسة

You might hear هي added for emphasis or contrast, but it is not necessary in the basic sentence.

It sounds very natural in Levantine Arabic.

A more Standard Arabic version would usually be:

• أختي في المدرسة

In Levantine, بالمدرسة is very common, while Standard Arabic more often uses في المدرسة for in/at school.

So the sentence you have is best thought of as a natural Levantine way to say it.