Pull-ups are just stupid, they're a waste of money and are so absorbant and comfortable you might as well just use plain diapers and save some money. They're nothing but a marketing ploy to trick you into potty training is easy or your kids will be trained like magic.
My daughter is 6 and potty training was the hardest and most challenging and frustrating thing I've ever done as a parent so far. I've also potty trained 3 boys at daycare too. I trained 4 kids and never want to do it again.
If she's potty trained or should be and has been you should only be using underpants or the thick cotton training underpants. Don't play around with pullups it's confusing and teaching your child it's ok to use a pull up or go in her pants. It's just sending a confusing message.
The only time I think pullups or diapers are appropriate is at bedtime(not naptime) when you have a newly potty trained kid who may have a accident or a child that has uncontrolable bedtime accidents. If their having accidents at naptime they're not physically ready to be potty trained.
With preschool if it's not daycare and an actual preschool I'd pull her out til she's potty trained a bit older prehaps, she's obviously not mature enough for it yet. Most "preschools" start at age3 and fully potty trained. It won't hurt to pull her out for a few months and try again later.
If she has a accident let her be fully responsible for it. You can help but she should beable to change clothing and cleanup. Teach her responsibility and that their are actions and consequences for everything we do in life. It won't be fun so she won't want to have a accident. It's just like many other things in life if you spill your milk at dinner, you need to clean it up ..yes it's a accident and not a big deal but that's the consequence for spilling.
What finally worked for me was telling my daughter she needed to be trained to attend preschool that coming fall, if she wasn't trained there would be NO preschool. She also got competative with the boys at daycare and they would fight over who got use the potty first and push and shove. I also stopped using pullups and did underpants if she had a accident I didn't rush to clean her up I let her be uncomfortable for a minute and come get me. Then I let her take responsibility and do the cleanup I helped but it was her job. She did it and was trained by like 30mos.old. Funny was she had been using the potty since she was 15mos. old. Just a very very stubborn, headstrong child.