A few suggestions - don't use plastic on the mattress, use vinyl instead. It doesn't make that crinkly sound and doesn't feel lumpy so easily. We bought a length of vinyl and found because it is so wide we had enough for two beds. Or to put it another way - we had a spare piece so we could hose off a wet one and give it an airing, while we had another to put to use.
I'd also be putting her back into Pull-Ups. Any school objecting - THEY can take her into their home for a few days and THEN tell you you can't. Maybe if you get a letter from her specialist explaining that if they don't use Pull-Ups then she is liable to need to be cleaned up.
If she objects to Pull-Ups - then she has a choice. I'd also be making her help clean up. This isn't a punishment, it's natural consequences. I did it with ALL my kids as they were being toilet-trained - they helped with the clean-up. It was part of learning personal responsibility. No sense of punishment at all, just simply working as a team to get the clean-up done. After all, I hadn't done anything wrong and yet I was pitching in too.
To clean up - first we removed the child from the environment that was soiled. If there was a fast spill to mop before it soaked into stuff, then I would throw a towel or something at it. If the child was clean, I would get the child to help me mop up. We'd get a bucket of warm soapy water and a couple of cloths. We'd mop with the cloths, wash them in the bucket, squeeze them out and mop some more.
If the mess is not going anywhere, I'd clean the child first. In the shower recess, clothes and all, then strip. That way any mess is contained to the shower recess. We had a telephone shower head installed so we could hose the children down easily. Really good for hosing down soiled rear ends. Soiled clothing - lumps got removed (either down the drain or tipped into the toilet). The clothing then got put in the washing machine, or into a nappy bucket (depending on quantity). Never let it dry out.
I always got the child to help tip out lumps and put the stained clothing into the nappy bucket or washing machine. I taught them how to use the washing machine.
Once the child is clean, I put clean clothes on them. I found I couldn't leave on any clothing items which had been worn when the "accident" happened - they still smelled, even if they hadn't been touched by excreta. The child does need to be smell-free, at least for a little while. They need to enjoy the clean smell and the clean feel, in order to learn to not do this.
With clean clothes on, it's time to clean the floor/bed/walls. Again, teamwork. Two people changing a bed is much easier than one. I've given the kids a spray bottle and scrubbing brush to clean the walls if they've been 'painting' them. With a cement-rendered brick wall, I can trust the kid to not do damage.
Urine is sterile. We tend to go, "oh, yuck," but it's not an infection risk. If left though, it will smell as bacteria in the air convert it to ammonia. You don't need to wear gloves to clean it up, although washing hands afterwards is a good idea because immersion in urea can damage your skin over time. To clean it out of carpet - we sometimes flush with about half a cup of water after we've mopped, to rinse it out. If any smell remains you can use various preparations including some essential oils.
As for clothing costs - I've been buying second-hand where I can. Nice new clothes don't get put on children who are likely to soil them.
And Star's idea of getting her to drink lots - that gives her plenty of opportunity to practice.
Helping to clean up plus wearing Pull-Ups - these are natural consequences. If she's still doing this as an adult, she will need to do exactly the same things so she may as well start getting used to it now.
With helping to clean up - I got my kids involved from about two years old. My sister did the same thing. I remember one day she found one of her sons rinsing out his soiled underpants. He had left it too late because he was too busy playing, so he came inside and started cleaning himself up, all by himself. She praised him and helped him from there.
But while ever you're being a martyr, announcing, "For heaven's sake!" and grabbing mops etc in exasperation, you're creating a spectacle she enjoys seeing, especially if she's angry with you.
The other thing you need to do - you must do other, more fun, tasks together too. That way she won't continue soiling just so you can work together on something. Instead, go bake a cake with her, or make biscuits with her. Let her have some choice in what to make or what flavourings to put in. difficult child 3 likes to make tiny biscuits with shapes, so I found a simple recipe and some small biscuit cutters. It's fiddly and tedious, but he feels some ownership in the result. And as long as he is doing the fiddly bits, I don't mind at all. It's like craft that you can eat. Any biscuit recipe that uses plain (all-purpose) flour with no raising agent can generally be used to make shape biscuits.
And here's a couple of tricks to try - make two lots of biscuit dough in different colours. You can substitute up to half the flour with cocoa powder, for chocolate flavour (I don't substitute more than a third). You can then roll out two thin lots of dough, cut them into the same size long strips and layer them together. Then roll up the two layers into a spiral shape. You should have a log whose cross-section is a two-tone spiral. You can keep this in the fridge, then carefully roll the log so it's slimmer, when it's about the right size you cut about quarter inch thick slices and bake them.
Or you can roll it out to about quarter inch thick, cut out large shapes then use small shape cutters to punch out a shape. You can then swap the punched out shapes so you have a dark star now sitting inside a white biscuit and vice versa. Or you can bake the biscuits with half with a small punched out bit missing, then when they're baked sandwich them together with butter cream or jam so you have a little window on one side peeping at the filling.
Or you can bake them with a cut out - but before baking, fill the hole with some coffee sugar or pieces of boiled sweets, and they will form a candy window (you need to use GOOD non-stick baking paper underneath though).
These are good things to do together, mother and child, as well as getting the child to work with you to clean up their messes. You need to do both.
I hope it helps.
Marg