Just read this thread and am horrifed 
I know its not typical but i worked in kindergarten at a local private school.
Kindergarten (3-4s) & Reception share their toilets, which have recently been refurbished to include a shower in the corner, (behind a curtain) for exactly this reason. There is also a bath mat and a couple of clean towels avaliable at the bottom of the spare clothes box, just in case.
We also had supplies of fragrance free baby wipes and nappy sacs, so less messy accidents could be wiped up if ok. (more often than not baby wipes were enough)
Now no staff member wiped childrens bottoms after a poo (even at just 3) , they had to do it themselves, but were always cleaned up properly if they had an accident.
If your son soiled himself at that school he would cleaned with wipes or even be showered if necessary, put in clean clothes, and gone back to normal lessons, and would NOT have been sent home.
I know this is not normal for state schools. At our school we also suncremed the children,
and the whole class shared a bottle of suncream
& the teachers applied it
(I realise this isnt common in state schools) (NB any parent who had a problem could of course provide their own sun cream for their child, but i didnt come across any parent who minded!)
As he is not yet 5 you dont legally have to send him to school yet anyway