Question

she goes to _____all girls school in the town.

Answer 1

She goes to an all girls school in the town.

all-girls school is a noun phrase

An article is used before a noun. There are two type of articles - definite and indefinite. 'The' is a definite article. 'A' and 'an' are indefinite articles. 'An' is used before a word which starts with a vowel. 

Answer 2

Thank you for asking this question. let me help you in n finding the answer.
The answer to be fill in the above blanks is "an".
So this makes the sentence like that " She goes to an all girl school in the town " 
This is because the phrase all girl is a noun.And there is also the use of an indefinite article which is "an" .Generally the indefinite article consist of "a" and "an" . But whenever there is a word starts from a vowel words the "an" comes before it.