Confused about clearing

So my first question is simple, when exactly does clearing start? On the 15th Aug? What happens if I call unis now?

My second question is a bit complicated:

With the help of UCAS, my understanding of Clearing is - that you fill out your application form and once you're finished, you'll get a clearing number which you hand over to unis. Then you'll get your offers or rejections. (regardless of 1st year or 2nd year of uni)

Now I'm at the reference part but my headmaster kinda turned it down (I'm looking to transfer to diff. uni for my 2nd year) by saying that my previous uni has to "release" me and I can only apply to 1 uni so I'd better make a final decision.
I must say though he's quite old and had the ucas system mixed up quite a few times because the ucas system has changed often in the past apparently.

So the 2nd question would be, is my headmaster right, or the first bit where I can apply to as many unis as I want (even as a 2nd year student)

cheers