Ok, today I went into assembley at my school and was feeling ill for about 10 minutes (shaking, dizzy, slightly disorientated), and then the next thing I remember was waking up and having no idea where I was for around 10-15 seconds, and not being able to see properly. According to my friend I was slumped forward shaking violently and making "weird noises" for between 30 seconds and a minute, and I have absolutely no memory of this.

I fainted once before, around 3 years ago, but this time was ALOT worse. Should I be worried about this?

Are you okay now? If so, then there is nothing to worry about.

I got my blood taken for tests last year, and I fainted and shook really badly (my mother thought I was dying haha) but I was OK straight after.

Uhm, that sounds like a seizure of some sort. I'd get that checked out ASAP. Unfortunately, getting tested for seizures can be pricey and ridiculously time consuming, so let's hope it's not that.

this is what i was going to say.
get to the dr's.

Go to a doctor and let them rule things out- they will look at the possibility of a seizure, low blood sugar, anemia (ie, do bloodwork), they will ask you about heart palpitations and maybe order some tests and echocardiogram to rule out it being a heart rhythm thing. They will rule out everything they can think of.

You can Google this, if they rule out a lot of causes they might consider it a vasovagal syncope, or a vasovagal response, which would just mean it's normal and not a serious health cause, it's just something that happens for various reasons. Googling will explain this much better than me. In my case my doctor became pretty sure I faint from a vasovagal response to pain because I have fainted a couple of times when I have had bad pain. It's like a nervous system response, and you have no control over it. You might be able to prevent a faint by recognizing the signs and sitting or lying down or whatnot. But it usually does not happen too often, so there is usually no need to take medication for it. If you start fainting a lot there is probably another cause. But googling it will explain a lot. The most important thing is to rule out other causes.

Yours almost does sound like a seizure though, seriously, get it checked out...hope it turns out ok.