Just adding in to this, not read the whole thing, so sorry if I repeat anyone......
My dd (5) has ongoing constipation/diahorrea issues from being born with an imperforate anus, and had a colostomy bag until she was 3.
I found Movicol very difficult to use, I never got the dose right, and dd leaked poo all the time.
Eventually her drs at GOSH agreed to try her on enemas. It's a faff and involves a catheter full of solution being inserted BUT it has been a godsend. It clears impacted poo, and they don't have any option but to go. We do it after school when I notice any mess, and it seems to work.
Next day I put a very discreet pad (you get them in the baby section of Sainsburys and are designed for children with bowel issues) which stops her leaking onto her pants. These can be removed, flushed and replaced. It stops the poo going onto pants/clothes.
I spoke to her teachers, and I have a set of pants/pads in the office. The school is very good over changing the pads if need be.
Dd went through a phase of refusing to tell anyone, thinking she would get in trouble or her friends would know, I spoke to her teachers, and we agreed that if she had an accident she would just go to the teacher and say she needed the toilet, which was a code for "I've done a poo"
I suspect that your child's skids are the same, ie very "sticky" for want of a better word, and simply wiping well with tissue won't cut the mustard, you need to use baby wipes. The school makes sure we have some in the disabled loo that she can use.
You do need to push a bit, as originally dds reception teacher would not have it that dds bowel problems were any different to any other child's, and didn't really get why I was pushing so much. However, dds bowel is VERY different to other childrens, and they now seem to have got it.
Sorry to have babbled, but I have spent nearly the whole of dds life discussing her poo, looking at poo and cleaning up her poo.
I think I'm almost an expert at it 
