An apprenticeship?
What we do is only really a semi skilled job, and in that I am being very specific.
Within a month or two you should be fairly adept at cleaning windows and using the various techniques.
As time passes you will get quicker and quicker at what you do.
As further time passes you will gain experience as you overcome fresh challenges that come your way.
If you are going to be your own boss then you have all the associated skills needed to run a business, and there are plenty of business studies out there that can be adapted for window cleaning.
And of course there are a couple of day courses out there that will sort you out by giving you basic training and advice......
But that is all you really need, an apprenticeship lasts for years. involves college, exams and so on, and that just doesn't apply to window cleaning.
Ian