It Was Inevitable, Really

The other day, my fifth grader's class was discussing parts of sentences.
One student guessed that a preposition was the subject of an example sentence, and my daughter gave a rather lengthy explanation about why that was not, in fact, correct.
I believe the sentence in question was something like:
The students went to the school.
"To the school is a location. It's where they're going to," she said. "So that can't be the subject. The action with the verb is where you find the predicate, and to...