For me, #2 is more common.
1) She said to me he was in hospital.
2) She told me he was in hospital.
Which sentence is more natural? Are they equally frequent?
If I were a native speaker of English, I would never shut up. :-)
They way I see it, "say to" is mostly used in direct speech.
I am not a teacher.
Interesting as the Ngram Viewer may be, comparing the frequency of two expressions that are not interchangeable has limited value.
"say to me" may be the only way to express a certain action, and "tell me" the only way to express another.
The fact that the latter is by far the more common of the two (which should come as no surprise to anyone) doesn't invalidate the former.
If you look at this analysis of two similar looking words you will notice a distinct preference for one of them, and there is no meaningful conclusion to be drawn from it.
I disagree. One can, as you did, select two words that have different meanings in different contexts. In that case, I agree with you. But the phrases under consideration don't have the same problem. Ngrams are not perfect, but they are far from meaningless. The issue was not about invalidating either expression, it was about frequency of usage.
The same relationship occurs with the past tense.
1. He said "Hello" to me.
2. He said hello to me.
3. He told me hello.
Would you really have told your students to use 3 instead of 2?
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.