I'm also unaware of an online yoga school that teaches siddhis. Unfortunately, despite what many seem to think, there is still a lot of stuff that is not available online and may never be.

The closest to an "authentic" yoga course online I'm aware of is one by Dr. John Mumford aka Swami Anandakapila Saraswati, with his Om Kara Kriya course(s). I took the first one many years ago but I don't think it is what you are looking for since, in line with most yogis views siddhis as at best a distracting side-effect, there is no explicit focus on them (though some relatively minor ones can develop from the work, there are faster ways IME and I would imagine you'd be disappointed - at least I would be if that was my focus in that course).

Another close but doesn't cut it option I'm aware of is the work which used to be at the site siddhashram.org but they seem to have changed the layout (and domain) since I last looked and I cannot find any of the sadhanas listed that they used to have (sadhanas for various siddhis included). It is primarily based on a magazine they send out (in Hindi but with certain sections in English). I have no personal experience with their work, but it was pointed out and recommended to me many years ago by a very able aghori I knew who vouched for their being genuine. Since it is tantric though, it by necessity requires the ability to pronounce mantra properly which, unless you're native in a sanskrit derived language, will require some minor linguistic training in phonetics.

There's another option which I can personally vouch for, but which is neither an online course nor as far as a layperson would know, yogic in origin. That is the work "Initiation into Hermetics" by Franz Bardon. The book is essentially laid out in a course format and siddhis can be developed following it. Despite being written by a European it is actually very yogic in practice and I'm aware of many of the same techniques or slight variations of them being used in more esoteric teachings of yoga/tantra.

So basically, I'm only aware of "close but no bacon" choices, sorry.

To answer your question yoga means union with god. siddhis are ability or skill developed in meditation when the meditator is getting to know him self. it is also said that siddhis should not be a goal it is view as hindrance to reach the final goal that is yoga. do you think that having more is less job or responsibility? and if you are to busy and follow your desire you loose the goal at hand. if I would compare this to a cup with water inside that as accumulated dirt and all kind of broken thing and poison as long as it is trap in the storm of the outside world the water is moving fast inside and you cannot see in that corrupt water but when the water is at rest not trap in the storm of the outside world the dirt go back down to the bottom of the cup and the worker can do is inner work and solve is division in truth. after all in what image or likeness have you been made? like it is said in the bible. But seek first his kingdom and his righteousness, and all these things will be given to you as well. http://biblehub.com/matthew/6-33.htm

look at Jesus did he follow course to get siddhis or to learn yoga or to be born again of water and the spirit. or did he go to the desert to meditate for 40 days? then you yourself should go to that desert a place where you are alone to do that inner cleaning to solve your division in truth. so that you can be born of water and can reflect the light of the spirit and being lead by the spirit to solve your last division so that the water and spirit can become one.The wind blows wherever it pleases. You hear its sound, but you cannot tell where it comes from or where it is going. So it is with everyone born of the Spirit." http://biblehub.com/john/3-8.htm

Let there be light, be the light that you can be, experience and enjoy.