dancergirl she spelt a few more words than 'unfortunately' wrong tbf 
I suppose it depends what age of child we are talking about.
The problem is that our language is very difficult to learn/use because of so many rules and so many exceptions to those rules.
I was only using my daughter's story to illustrate a point. I don't think she would have attempted to use some of the words she did if she'd been concerned about getting it 'wrong'.
It is quite possible that if the teacher realises the children have got their theres, theirs and they'res mixed up, then she would take an opportunity at another time to address these as a whole class.
Perhaps in the first half hour of the day during the register and children arriving or before the end of the day, or when things are cleared away and there are a few minutes before lunch. Those are all the times I used to play phonics games, or give spellings work etc to reinforce the learning little and often. And it would be quite easy to devise 'games' involving (for example) there, their and they're and then encourage the children who find them difficult (quite a few I would imagine) to participate.
That's more productive than just crossing outs etc in books because that is not going to make the child remember the correct use of punctuation/the correct homophone because it's ineffective - the child will glance at it, not commit it to memory and it has moved on. In fact, all it does do is reassure the parents that the teacher has spotted it.
The reasons that a lot of the approaches used when we were younger are not used anymore is because they don't work. It's hard to move away from those ideas because we almost see them as the right or the only way to do things properly.