Potty Training and Pre-school

My DD is 2.5 and wearing panties the majority of every day and night, but she is far from potty trained. She has proven that she has continence, meaning she can hold it through naps and nighttime. She will go sit on the potty, immediately stand up, want to wipe though nothing was done, get down, then go pee in her pants. She demands a diaper to poop, which I don't deny because if I do she'll hold it in and we already battle constipation due to a picky diet. Every 2-3 days she will have a great period during the day and will go potty (#1 and 2) successfully and gets excited about being a big girl. I don't know what to do to encourage her to actually go in the potty more. I sometimes wonder if she has trouble with releasing her pee and poop. Am I reaching here? Is this just a phase?  Help me. She has to be potty trained to move to the 3 year old class in the fall or  do any of the fun classes (dance, etc.).