THE school population in Wales fell by 100 to 499,800 last year, reversing the upward trend of the past decade. But the number of full-time teachers rose from 25,250 to 25,400 in 1999-2000, with anincrease among women and a slight decrease among men. The changes produced improvements in pupil-teacher ratios, from 22.3 to 21.9 in primary schools and from 16.7 to 16.5 in secondary.