It’s for a 6th form prize-giving ceremony, held in the evening, dress code is for the students.
Can’t decide if it means formal, as in boys in dinner jacket-black tie and girls in glamorous full length dresses, similar to prom.
Or smart, which I would interpret as boys in suit/jacket and tie, girls in smart skirt and blouse, trouser suit or corporate-looking plain dress perhaps with a jacket.
Or something between the two, which would be..?