Dear Mr. Nabil
I think the correct answer is WHICH not where
In your example "SCHOOL" is the direct object of the verb ATTEND
We say: attend the funeral / class / a meeting ....etc.
We also say: Which school do your children attend? -
- The school which my children attend is called Al Fustat
Here is An Easy Test to decide whether to use WHICH or WHERE
Use the word replaced by the relative pronoun after the verb that comes after it
1. If that word requires an adverb particle "e.g. in, on, at, under, below ....etc. before it, then it refers to place or position. In this case we must use WHERE
Example
The place (which / where) he was born is Tanta.
Note: He was born in this place./ in Tanta / in it
Here the correct answer is "WHERE"
2. If that word does not require an adverb particle, it is a direct object of the verb preceding it. In this case we have to use WHICH
Example
The place (which / where) we visited last week was fascinating
Note: We visited that place. We visited it
In this case we are using `place` as mere object. So we must use "WHICH or THAT"
BEST REGARDS