I cannot remember the method's name but this is what worked with my son...Our therapist placed a chair in the bathroom. My son would sit in the red chair while working/playing on the ipad/puzzles etc, all while drinking apple juice/water (just loading him up with fluids) Every 3-5? minutes or so they would move him to the potty and wait for him to try and use it. Positive Reinforcement "Yay, that's using the potty!" Make it a party every time they use the potty! They did this for at least an hour every morning. After the hour was up they would frequently take him to the potty. Once he had several successes, they started moving the chair further away, 1ft,3ft.etc. After 15 feet they got rid of the chair and started playing down the hall. BAM!! Potty trained!
Definitely make sure the school is on the same plan as you in regards to potty training and if not have it written into the IEP. It was hard with my son potty training he is now 6. I was worried it would never happen but this psychologist told me about positive reinforcement and it was hard but worked. Also a chart with stars is good.
I posted up a step-by-step picture schedule on the wall in the bathroom. She started following it, she is now 6 and has very few accidents.