A Brazilian judge has ruled that the results of the country’s university entrance exam be annulled because of security issues and problems with some test papers. The Education Ministry will appeal the decision, but it could mean that millions of students will have to retake the test, throwing Brazil’s school year into potential chaos. An estimated 2,000 or so papers administered this month were printed with errors that confused students. Reports that some students were allowed into exam rooms with pencils and watches, both of which were forbidden by law, were further evidence of lax security, the judge said. The problems are just the latest in a tumultuous two years since the test was revamped to democratize entry into Brazil’s publicly financed universities.