The school's deputy principal, Sabani Amjuye, said a candidate stepped down because insulting comments were written on his poster.
His supporters then started a riot, which involved bows and arrows, sticks, stones and knives, that lasted an entire day.
Mr Amjuye said school facilities were damaged, and three staff houses were razed - a fire in the boys' dormitory managed to be put out.
He said the decision was made to suspend classes indefinitely because of lingering tension.