A schoolkid is a child who goes to school. It is a friendly, informal word — the kind you use in speech, in newspapers, and with a smile, not in a very formal letter. Kid here just means child. Schoolboy and schoolgirl name a boy or a girl; schoolkid does not. It usually means children in primary or secondary school, not university students, even though those students also 'go to school' in a loose way. Adults say the schoolkids when they mean the children as a group, often from outside: the ones on the bus, in the playground, filling the street at three o'clock.