The annual Independent Schools Council census was predicted to reveal a slight rise in rolls, despite a demographic downturn in the school age group and above-inflation increases in fees of up to 8 per cent in the most expensive schools.
The figures were also expected to highlight the continued rise of co-educational boarding.
Thirty years ago, there were 24,000 girls boarding at single-sex schools, 12 times the number boarding at their co-educational counterparts.
But last year, for the first time, there were fewer boarders at top girls'
schools than top mixed schools, a gap that was expected to widen this year.
The trend has increased as public schools such as Marlborough, Wellington and Rugby have begun to admit girls.
But Frances King, head of all-girls' Heathfield St Mary's school in Ascot, argued this week that boarding with boys can damage girls' self esteem during puberty.
"A single-sex environment is an unthreatening refuge where adolescents can cope with physical changes without worrying about what the opposite sex thinks, and they can develop confidence and take risks without having to suppress them for fear of not looking cool in the classroom," she said.