Does saying or writing the Shahada convert (revert) someone to Islam?
To convert to Islam, you say the shahada. That happens to be the "calligraphy" phrase kids were asked to copy.
Shahadaa is not just merely saying words or writing them. It's a matured decision to understand Islam whole heartedly without any compulsion and uttering those beautiful phrases. (Some people even say it out with tears of happiness).
Simply saying those phrases without meaning it from your heart doesn't revert anyone to Islam. But still, these words are considered holy and primary faith a person should have to be a Muslim. So please try not to misuse it and use them at unholy circumstances..
I'm not sure why kids have been asked to write them in calligraphy classes though.
Shahada by definition means "to observe, witness, testify". Testify means to make "a statement based on personal knowledge or belief". Merely saying or writing
"I bear witness that there is no God except Allah; And I bear witness that Muhammad (peace be upon him) is His servant and messenger"
without believing in it doesn't count as Shahada. Wikipedia writes:
The Shahada (Arabic: الشهادة aš-šahādah "the testimony"), is an Islamic creed declaring belief in the oneness of God (tawhid) and the acceptance of Muhammad as God's prophet.