As someone who was a child with disabilities in a mainstream school, I thought I'd jump in with my point of view.
My disabilities were sight/hearing losses - which actually are not obvious disabilities at all (perhaps complete loss might have been, but partial not).
I would not have appreciated a teacher giving people information at all. Not least because teachers are no more clued up than anyone else about individual disabilities, and judging by comments they made to me, I think they would have been quite misleading if they'd favoured other people with their impressions!
Perhaps another child might think differently. But it's up to the child. If they want it explained, they can do so. (obviously harder at a young age, I know.)
Some children might have behavioural difficulties at school because of their family situations - but is it appropriate for the teacher to tell the rest of the class that 'X's parents are splitting up at the moment', or 'Y's dad beats him up'. I think not. Equally not appropriate for someone to take it on themselves to 'explain' a disability.
Throughout life, your child will meet people with disabilities (hidden or otherwise), or with various other issues in their life. This is the time to start teaching them that everyone is different, and you don't jump in to judge other people's behaviour.
Also 'knowing' that someone has a disability, or what the name of that disability is, or even various information about it, unfortunately doesn't magically make everyone (adults or children) behave well around them. Often the 'knowing' causes problems of its own. People use it to be vicious, or try to be helpful and over-compensate, or all sorts of other things.
The best thing is to get to know the person as an individual. It will gradually become clear through small daily interactions what is appropriate to do / not do (just like with any other person, actually). And if you don't want/need to get to know them, then you probably don't need to know any personal information about them, just withhold your quick judgements. I think that's what we should be teaching children to do too.
frogs advice about what to say looks good.
Just my point of view.