I still don't understand how a concept of a masturbation tutor or instructor would even be a thing.
Do 10 and 12 year olds seek help in this area today? Do they ask each other if they have started playing with themselves yet? They sure didn't when I was that age.
You say things would have been better had someone instructed you prior to turning 15. How would that play out in reality? Do you wish a friend had asked you about your masturbatory habits when you were 12 or 13?
I'm not trying to be rude or pry into your childhood. I just can't even wrap my mind around the idea of a middle school kid seeking help with masturbation techniques.
How can anyone ask for help about a practice they have no knowledge of? Of course kids don't ask their friends such a question. How could they conceive of doing so?
If you've read many of the accounts here, you will notice that it is usually the person with the knowledge about masturbation who takes the initiative and instructs on how it is done. With kids of school age it was often an older friend, fellow student or maybe family relative like a cousin. Or maybe even an adult, an uncle, instructor of any sort, a supervisor at places where undressing occurs. Heck, maybe even a religious figure or Boy Scout troupe leader.
For myself, my experiences, my family moved to Belgium when I was 13 and I was still ignorant of many things. Not anal insertions, that I had been doing for many years already. But not masturbating. Maybe I am being uncharitable, but my schoolmates in Belgium were real wimps, goody-goodies. Not that I had many friends then anyway.
In retrospect, I suppose had we stayed in the US someone out of the many dirty-minded kids at school would've showed me the what and how. Or maybe had I stayed in the local YMCA one of the supervisors might have. Who knows? I rather wish I had been instructed/showed when much younger.
And it wouldn't have mattered much who did so either, I now think. To think of all the fun I could've had in those missed years.