One time a teacher looked through my personal journal...?

Unfortunately yes, a teacher can go through your personal stuff if you have it at school. If you have been looking at this site, you will see a lot of postings about teachers going through phone messages, backpacks, lockers, and the like. Courts have generally ruled that the schools (and the teachers as agents of the schools) have the right to go through your things. They can't strip-search you, but they can look through a book.