Teachers would initially be resistant to change. Some might be left without a subject to teach in the afternoon. Well, that is the whole purpose. Some teachers would be required to teach their academic subject since there will be some students that wish to focus on mathematics or a language, perhaps one teacher from each department. The remainder of the teachers would be involved in running the various programmes. At first, a teacher might find it difficult to run a tennis training programme, but in time, since teachers are experienced organisers, they would soon become very proficient.
Not enough academic matter
Juniors would study 3 subjects every mornings: Maths, English/Social Studies, and Science. An extra subject could be added (tech / art / drama / language etc).
Seniors would study three subjects of their choice each morning. Maths and English recommended for Year 11. Depending upon the type of timetable, a fourth subject could be added. However, studying three to a high degree of proficiency is the preferred aim. If a student were only keen to do one or two morning subjects, that could be accommodated if they filled their free time productively.
The whole point of this programme is to reduce the 'total' academic nature of the school programme and to get the students motivated for excellence in their chosen areas. For students who wish to purse an academic future, they do have the opportunity to study a subject of their choice intensely for a year in the afternoon. In fact, they ought to be able to cover two or even three years of material over the course of one year of intense study.
This would pose a lot of trouble at first as managers and teachers would have to learn to forget the past and even ignore current university requirements. The aim is to create more rounded and self-confident high school graduates. If it is high academic achievement across a broad range of subjects that is required (the norm) then a parent can send their child to a different school. Universities would no doubt recognise the benefits of this programme in due course.
There will, no doubt, be many more issues that educators might raise ... but all can be overcome with positive purposeful determination with the single aim of improving the student mind.
Improved sports results, excellent musicians and other artistic performers, improved results in chosen academic subjects, increased motivation, increased self-confidence, increased ability to cope with the outside world after leaving school.