An IEP plan, they are legally required to follow it, which could be good but they will be very reluctant to put a lot of specifics in it because they don't want to get themselves in trouble if they happen to lack resources or organization at a particular point in time and can't follow e.g. they have a budget shortfall and have to fire the aid. In most schools there is a fairly narrow definition of who demonstrates enough needs for an IEP, and it requires a lot of testing beforehand. With a 504 plan you have more flexibility, not so many legal requirements which can sometimes mean things can fall through the cracks if the teachers do not have good support to keep the plan going. I have taught at 3 different schools and one did a wonderful job with writing and keeping very helpful 504 plans and the other two did not do well with it at all.