Sorry to rant. Last week I was at college in art and it got to much I'm shaking, crying stinking and just babbling (I was having a meltdown).
A member of staff told me to stop since their was only 2 minutes left of the lessons. As most of you will know you can't delay a melt down her saying this just stressed me out more and sent me further in. Don't tell a person to stop having it meltdown because it's inconvenient and met offend someone.
The bolded words in your quote struck me first. I am able to delay a melt down and others on this site have been able to do the same. That does not mean everyone can do this, of course. Saying that it is impossible, pretty much guarantees that you will not be able to do it. Having a meltdown in public is a bad idea and not safe. Learning to delay a meltdown is a good skill to have, so I think you should stop psyching yourself out by declaring it as impossible.
You are not certain to be able to learn to delay meltdowns, yet. Once you change what you tell yourself, at least you have a chance.
If you think about the times you have had meltdowns, you can figure out what they had in common. It will probably help to write down what happened when you had a lot of different melt downs. You need to know what happened before, during and after the meltdowns. Then you need to compare them and see what is the same for all or most of them.
The first step is prevention. What happened that triggered your meltdowns? Once you know that, you can figure out how to avoid as many of those triggers as possible. You mentioned that this particular meltdown occurred because you were in art class. I am picturing you in art class where you get your work critiqued. There is not enough information in your post to be certain, however. I took art classes in college, and critique time was way the most stressful part of the whole class.
You can not avoid getting critiques for art classes, but you can do some things to make it less stressful. You can ask for critiques from fellow students and the teacher ahead of time. If this is too big a step to start off with, you can imagine doing it and keep playing this over in your head a while to get yourself used to it. Once imagining it gets easier, then you can try it with somebody you are comfortable with, even someone who is not in your class.
I do not know about your school, but most of them do make an effort to accommodate disabilities, and are even required by law to do this. Very few schools do not get some sort of government funding, which means they have to accommodate disabilities. If you have not wanted to pursue this in the past because you didn't want to "out" yourself as having Aspergers, you might as well forget that. Your public meltdown has now outed you as having a problem. It is a bad idea to leave your teacher and the school administration wondering what was going on with your meltdown. An explanation to them will probably be helpful to you in the future.
Once you analyse your previous meltdowns, you will be armed with information on what it will take for you to avoid a meltdown trigger. This will give you a chance to think of what you might ask for from your school to help you avoid your triggers, or at least lessen the severity, and give you a chance to go to the bathroom or whatever else will let you get it over with.
Not letting you leave the room was clearly a bad approach, and it sounds as if the school was violating laws by doing that. I do not know if you want to make an issue of taking any legal action. That would be stressful for you, and a last resort if they refuse to help you avoid more meltdowns in the future.