英文 Tense

In this sentence, there are two options for the blank.
1) kept
2) had already kept
If you want to say that when Miss Lau came, the pupils just kept quiet at that moment, then 'kept' should be used as this is only a simple past tense sentence.
But if you want to emphasize that before Miss Lau came, the pupils had already kept quite, this past perfect tense should be used.
Do remember, English sentence is extensive in expression. A sentence can differ from meaning if you change only the tense of the verb.

2007-10-13 20:06:08 補充：
For the 2nd sentence, there is only one option.When the test comes, the pupils have to study hard. As this is a fact that no longer changes in the future, the tense here ought to be simple present tense.

kept.

Because is past tense, so we used kept in this mata