Students always have many scheduled tests, but some professors like to give surprise tests to students. Please talk about advantages and disadvantages of surprise tests.
I think surprise quizzes can be a disadvantage to both teachers and students. As a student, there’s nothing worse than being reminded that you haven’t been on top of your studies. Even the most hard working student needs a little heads-up about quizzes and so it can be disheartening to students and an inaccurate assessment to teachers about who is a top student or not.
With regards to the advantage, pop quizzes can ensure students are kept on their toes and always prepared. If students know there might be a chance they’ll get random tests, they won’t slack all semester and only study towards the end. So, there are definitely 猜词互动（利与弊）.
a warning message
disappointing, frustrating, dispiriting
e.g. It can be disheartening when you apply for a credit card and get turned down.
be on toes:
remaining to be alert and ready for anything that might happen
e.g. His fiery campaign rhetoric has kept opposition parties on their toes for months.
work less hard than you usually do or should do
e.g. He had never let a foreman see him slacking.