King's had previously taken girls at Year 12. It announced this year it would now welcome female applications at Year 11 and has been heavily marketing the decision through traditional means and extensive social media campaigns.
The school said it made the decision "to enrich the experience of girls at [the school] and their contribution to the Senior School".
The decision came as something of a surprise to the sector, where most believe it was a move to bump up the school roll - which, like all independent schools, took a hit in the wake of the global financial crisis.
Diocesan principal Heather McRae said up to 27 girls from both Year 10 and 11 had indicated they would switch to King's next year. She said while it was sad to lose students, the school was not concerned about the impact.
"We are confident in what we deliver and believe in a girls' environment, and we have a healthy roll," she said. "Our school is a great opportunity for girls."
St Cuthbert's head, Lynda Reid, said the same number were expected to go from her school, although both women said some families were considering changing their minds after the surprise announcement of a change in leadership at King's.
Its principal, Mike Leach, announced his resignation this week. He will be replaced by current Takapuna Grammar School head, Simon Lamb.
"There has been a pattern of students leaving since King's started taking girls, but usually it's about half that number," Ms Reid said. "We predict it will settle down next year."
Ms Reid said the school's roll was also healthy, and predicted to be higher again next year. She said there was a growing interest in girls' education. She put the higher numbers of students changing schools down to a heightened awareness of the importance of education.
St Cuthbert's itself had a large intake in the senior years, as students from state schools wanted to participate in its NCEA and IB programmes, she said.
Baradene College would lose three students to King's, its principal Sandy Pasley said. Two of those were on sports scholarships. King's, which offers a range of scholarships just for girls, did not return calls.
Diocesan, St Cuthbert's and King's have all seen roll decreases since 2011 (although none have been major) while St Kentigern College is growing.
Annual fees
King's College
Year 9-13
Roll: 936 (120 girls)
Fees: $24,093
Diocesan School for Girls
Year 1-13
Roll: 1303
Fees: $19,900
St Cuthbert's College
Year 1-13
Roll: 1369
Fees: $20,656
St Kentigern College
Year 7-13
Roll: 1800 (721 girls)
Fees: $19,000