To determine which sentence contains the verb in the third person plural in the simple present tense, we must evaluate each option individually while understanding the rules of subject-verb agreement and verb tense usage.

1. The question is about identifying the correct use of a third person plural verb form in the simple present tense.
2. In standard English grammar, the third person plural refers to subjects like "they," and the simple present tense form of the verb remains in its base form (e.g., "walk," "bring," "are").
3. Let's evaluate each option:
   • Option 1: "Are they here in school now?"
     - In this sentence, "they" is the subject, which is in the third person plural.
     - The verb "are" is in the simple present tense and matches the subject in number, making it a third person plural present tense verb form.
   • Option 2: "They have walked to school."
     - Here, "they" is the subject, which is indeed third person plural. However, "have walked" is in the present perfect tense, not simple present.
   • Option 3: "Are you not in school yet?"
     - In this sentence, "you" is the subject, which is in the second person. Thus, it does not match the requirements of the question as it's not third person plural.
   • Option 4: "He is here in school now."
     - "He" is the subject and is in the third person singular. The verb "is" is also singular, which does not fulfill the third person plural condition.
4. After analyzing all options, Option 1 ("Are they here in school now?") correctly uses a third person plural subject ("they") with the simple present tense verb "are." This makes it the correct choice.

Conclusion: The sentence that contains the verb in the third person plural in the simple present tense is "Are they here in school now?"