I think "In December, school is closed" should be accepted. I'm not sure about in Hindi, but it is fine to move the prepositional phrase (in December) around in English. Also, since there is no word for "the" in Hindi, saying "the school" and just "school" should both be accepted.
In English that could be referring to a specific school or schools in general. In the hindi case if it was schools in general then the statement would be होते हैं. होता है is singular.