Not only, but also (Verb Form)

I'm confused with this rule: 1. If one subject is singular and the other is plural, and the words are connected by the words "or," "nor," "neither/nor," "either/or," or "not only/but also," use the verb form of the subject that is nearest the verb.

And I came across this question: Not only the students but also the teacher (was, were) anxious to finish the lesson.

If the above rule is followed, the answer is WAS. However, if I use http://www.reverso.net/spell-checker/english-spelling-grammar/ to check, the answer is WERE.

Which is correct? Please enlighten me. Thank you.

In the particular sentence in the question, the subject unambiguously consists of multiple people.

It can be paraphrased this way:

They (all of them; not only the students but also the teacher) were anxious to finish the lesson.

It's simply the fact that it's stylistically awkward, and it might be a good idea to rephrase it:

Not only were the students anxious to finish the lesson, but so was the teacher.

Other specific constructions, which have a more ambiguous subject, would be more amenable to the "nearest to the verb" rule mentioned. But, at the same time, also better off rephrased.

If you are confused by the way something is phrased, and you are forced to pick something that a rule says is right in order to resolve the ambiguous situation, it's often better to simply fix the sentence so it's not confusing in the first place.