Well done you!
Little surprised the headmaster asked you to do it infront of the entire school! sounds like he might be the first person that needs a little more understanding of the said brain type!hehe... But aside from that, yes, maybe just speak about yourself and how you feel and see things, how it can effect you, the situations that everyday people take for granted which for you can be an enormous challenge mentally and physically..and what people see on the outside is not necessarily the reflection of the inner. This is a perfect moment to advocate the spectrum, as mention removing the stereotypes, ie lack of empathy etc and back this up with clippets from recent university research showing these public believed theories have been wrong all along..and if people see children screaming in a shop or a mall that just maybe they are not trying to be awkward but extremely overwhelmed by their surroundings, and maybe mention some things positive about anything that you can seem to do better than others, but its about a partnership, a mutual understanding.. This information will stay with them into adulthood where they can then give their own children the right information if ever it came up in discussion...Changing the mind frames of almost entire populations is a slow progress, but these wrongful ideologies are past down to the next. So you are an awesome advocate to help get the train rolling
I dont want to say too much as this is a very personal thing seen from your eyes and therefore should be what comes from your own heart!
Best of luck 