Unbeknownst to you, many of your classmates are fasting from morning's first light to sunset. They are observing the Muslim month of Ramadan.
Ramadan, says Wikipedia: (also Ramazan, Ramzan, Ramadhan, Ramdan, Ramadaan) is the ninth month of the Islamic calendar. It is the Islamic month of fasting, in which participating Muslims refrain from eating, drinking, smoking, and indulging in anything that is in excess or ill-natured; from dawn until dusk [1]
Fasting is meant to teach the Muslim patience, modesty and spirituality. Ramaḍān is a time to fast for the sake of Allah, and to offer more prayer than usual. During Ramaḍān, Muslims ask forgiveness for past sins, pray for guidance and help in refraining from everyday evils, and try to purify themselves through self-restraint and good deeds.
As compared to solar calendar, the dates of Ramadan vary, moving forward about ten days each year. Ramadhan was the month in which the first verses of the Qur'an were revealed[Qur'an 2:185] to the Prophet Muhammad.