I think that of all the factors that impact on future employment/success the type of school is the least important. More influential factors are the child's ability, their motivation, parent support and parent belief in the importance of education.
I went to a public school with a poor reputation but it was always assumed by my parents that I would go to uni. My friends at school were the more academically inclined students. From my friendship group we have; a dietician, pharmacist, psychologist, high school teacher, accountant and an anaesthetist.
That said my DD will be going to a private school but only because DH is Catholic and wants our children to go to a Catholic school.
Personally, if I were in your position I would be inclined to use the money for tertiary education rather than a private secondary school. And also making sure you reinforce the importance of education, listening to teachers, completing schoolwork and doing the best they can.