When a teacher uses a fixed interval schedule for quizzes (e. g., every friday), this usually encourages students to

The correct answer is that "it encourages students to avoid classes during Fridays."

Due to the fact that teachers gives and prepares a fixed-interval for quizzes in their subject areas, it would be more predictable by students that every Friday, they will be having their quiz, which makes them want to skip classes. Which is why it is better to have random interval in giving quizzes or to give a surprise quiz.