Does the Hadith: "Whoever usurps the right of a Muslim by his oath, Allah will ordain Hellfire for him and forbid Paradise for him" apply to taking a tissue from a classmate in the sixth grade of primary school, and is it considered stealing, and what should be done to expiate this sin?

The hadith (definition) mentioned in the question does not deal with mere theft, but rather involves consuming a Muslim's property unlawfully, coupled with the audacity to take a false oath (al-yamin (definition) al-ghamus). Regarding what you did by taking the handkerchief from your colleague: If you were below the age of puberty, there is no sin upon you. If you were an adult and knew of your colleague's consent, there is no blame on you, as some jurists hold the view that it is permissible for a friend to take from a friend's property if their consent is known, based on the verse: "Or your friend." However, if you doubted her consent, or knew of her non-consent, then your taking of the handkerchief is forbidden, and you are obliged to repent and seek her forgiveness. A tissue is a trivial item of no value, and taking something trivial is not considered theft, even if it is fundamentally forbidden. A person is only described as a thief if the stolen item reaches the threshold for theft.