In your comment you provide extra context to the sentence heading your question, namely:
- Soon the hen was surrounded by animals who wanted to see the strange
I interpret the word soon here as indicating the state of the hen on completion of the action of surrounding by the other animals. And a state past particple is adjectival, not verbal.
Contrast this with the following alternative sentence:
- One by one the hen was surrounded by animals who wanted to see the
In this case the ongoing action of the surrounding is being described, and the past participle is part of a passive construction, i.e. verbal.
Finally, a comment: I don't think it is unreasonable to ask young students to identify parts of speech. But it seems unfair to give them decontextualised, ambiguous examples such as "The cat was surrounded by mice."