Which of the following is grammatical?
- Is there a best school?
- Is there the best school?
I'm sure that the first one is right. But why?
The is the definite article. A is the indefinite article.
The question "Is there a best school?' is, by its nature, indefinite. Ergo, a.
It is necessary to determine the meaning of the question. Does it mean
Without more context, most native English speakers would interpret the question as in number 1. In that case, the declarative form of the sentences would be
This is an adverbial use of there meaning
The sentence means that a best thing exists, but we are not indicating which particular school is best. The reference is indefinite as to which school merits the title, so the indefinite article a is used.
If the intended meaning is that described in number 2, the declarative sentence would be
In that case, there is used as an adverb meaning
If that was what was intended, a particular school is being identified, and the definite article the would be used. However, most English speakers would not use this construction.