A new two-storey science block will be built at Wolverhampton Girls' High School (WGHS) by building on its playing fields, as part of a £3.5 million investment.
The cash boost, from the Department for Education's Selective School Expansion Fund, was secured on the conditions the money was used to provide spaces for disadvantaged pupils – and for outreach projects.
But planning officers said the move would see two out of six tennis courts, which lie on the green belt, built on at the Tettenhall Road site.
A report said the application should not be approved except in "very special circumstances" – which the school had met with its push to help disadvantaged students, which outweighs the harm to the green belt.
It will see 175 more pupils attend the site over a five year period, between Year 7 to Year 11 – with numbers jumping from 145 pupils to 180 per year group.
Trudy Young, headteacher at the school, said at Wolverhampton Council's Planning Committee: "As a Wolverhampton girl I know the impact that our school has on young girls' lives.
"I want to ensure more Wolverhampton girls aspire to come to the school and we're seeing a real increase in girls taking the exam and securing a place.
"The funding can only be delivered if we carry out the outreach work and only if the extension goes ahead as extra accommodation for the extra girls.
"It will benefit the city."
Work will be carried on the remaining four tennis courts in a bid to make up for the loss, with the facility set to be opened up to people in the area.
Former mayor of Wolverhampton Councillor Phil Page said the plans were "good for the school and for Wolverhampton".
Councillor Keith Inston, chair of the planning committee, said: "I do think it's a good application and if it benefits one disadvantaged youngsters, it's done its work."