English grammar

The notice said that campers must clean the site themselves.

or

“The notice said that campers must clean the site ourselves”said a student.

In my own opinion, i think that you are right and your teacher is wrong, sorry to say that.
campers in this sentence is the 3rd person plural, and ourselves is a word of 1st person plural. The two words are formed in totally different situations.
Themselves, however, is 3rd person plural and matches the word, camper, perfectly.

Firstly , I appreciate you have make use of the oppurtunity to ask in yahoo knowledge.
You should ask your teacher about this to confirm since your teacher told you like that.
and that is a kind of learnning, attitude,
Both are correct,
If your teacher said it is wrong he /she may have their reason go and ask them.
And You have to look at the question.
What is the question asking?

your teacher takes it as first personal narration....and i think Themselves is all right~!