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.