I would say Education Direct.They've been around forever.BUT.I had a guy in my shop the other day asking the same thing.Why not buy a used/junk/cheap bike and the manual to go with it?I feel there is no substitution for hands on.I went to school for aviation maintenance.The book work was to understand the function of a paticular part(ie.piston).But until you see/feel it,take apart reassemble you're not getting the whole thing.........Just my opinion.

I understand.My Ex took a class,I think with Penn Foster.It was ok.They send you a section at a time.You do it,take the test and then when you pass that one they send you the next one.I don't think she ever got her "diploma" though,and all the money was paid. I forgot sometimes people aren't born into motorhead families,lol.
I'm the kind of person that learns hands on........which I still have both of,with all ten fingers.

How about buying a clunker & fixing it up? Get a book,
or,, is there a local, private shop around? Maybe you could volunteer some time, clean up, roll bikes in & out, & get some training.,

Sometimes it takes time to develop a relationship with an owner.. Go hang out, take do nuts, visit a while, pick up a broom & sweep a bit up.. I got a job in a blacksmith shop that way & that guy had never had anyone work with him.

Ask around & see if you can find someone who repairs/rebuilds bikes for people. You'll probably find the kind of guy you need from word of mouth, because they don't tend to advertise & generally work from a shop at their house & usually work alone. Get to know him before you ask about learning or helping out, because he's heard it before & won't trust you until he knows you a while.

If you're lucky, you'll find someone like I did at age 14 & learn lessons that last a lifetime. Even though it was cars and not bikes, the same things apply.

I still remember:  Son, that engine doesn't know who made it or how many cylinders it has. It just needs air, fuel, compression & a spark at the right time & it will run.

That has led me to work on some unusual engines through the years. And what I learned is still true today. And my daddy would bring home broken things for me to take apart because if it was already broken I couldn't hurt it. I got my first lawn mower that way, was given one almost new with a broken rod that I replaced.