OnceWasMummyPig,
I do actually have a child and she has many problems of her own, although, admittedly, not potty trainig. And I view these problems as my own (and our family's), rather than the school's and think it is first and foremost my responsibility as a parent to find the most appropriate and comfortable (for my DD) way to deal with them. Not the school's.
I totally sympathize with OP and with that particular situation - the school most definately should have called her.
However, I do have an issue with the expectation that any individual teacher SHOULD do anything about it. A teacher's job at a mainstream state school is to deliver the curriculum appropriate to an AVERAGE child in the classroom (as that's what the curriculum and mainstream provision is supposed to deal with).
Any individual teacher in the much hailed (on this board) role of loco parentis is supposed to be able to deal with emergencies (within school policies). An ongoing toiletting issues is not an emergency and bringing in the in loco parentis arguement into this is not appropriate.
Anything beyond this is the parent's, school's and LEA's responsibility to sort out, whether to have an IEP, whether a TA, a care assistant, a parent on standby,etc. It's most certainly not a job of the class teacher.
With regards to "delayed potty training" debate, I don't think that the parents are lazy, far from it. But sometimes, children develop at different rates (i.e. it's not a physical medical problem) and "crack" potty training later than the average range for teh peer group. If that's the case, yes, I don think it is significantly more beneficial to deal with this problem first before putting the child through recpetion with 30 kids, 1 teacher and a day from 9 to 3pm (on average).
It's like when a kid has speech problems, i.e. mixes up sounds, mispronounces a lot, etc. There is absolutely NO point trying to teach such kid literacy in mainstream, average reception classroom until the speech problem is resolved. It will be totally futile.