At my school I have learned - linking verbs are followed by adjectives but the following sentences confuse me. 

1. > 
> He is a good teacher. 
> 
> 
>
2. > 
> He became headmaster.
> 
> 
>
3. > 
> She is a nurse.
> 
> 
>

In the above sentences Linking verbs are followed by noun and noun phrase.