Good morning, Bamthor.
(1) As I type, no one else has replied. If I start, maybe others will join in.
I REALLY want to know the answer.
(2) Here are some thoughts -- NOT answers!!!
(3) You are tutoring a 7th-grader. I imagine that a 7th-grade teacher
does not want to make grammar too difficult:
The teacher: What does "there" mean?
Sammy: At that place.
The teacher: Excellent. And what part of speech is "there"?
Mary: An adverb.
The teacher: Perfect. And what word does "there" modify (belong to)
in our sentence?
The teacher: Why?
Mona: Because "were" is a verb, and adverbs modify verbs.
I imagine that if a 7th-grade class could have this level of discussion, any teacher would be delighted.
I, however, am not satisfied:
(a) I feel that an adverb cannot modify a linking verb, since a linking verb means nothing.