The other requirement will be the exams required by the individual state. I'm in Texas so took written exams for general regulations pertaining to pesticides, an exam for aerial application and another for an area of specialty, such as range and pastureland or field crops or something like that. I hold a commercial aerial applicator licence from Texas Department of Agriculture.
If you don't have these exams you can work under the direct supervision of the company you are working for, although most companies will require you to have them.
Many of these exams reciprocate from state to state, so you will probably only have to take them once.
I don't know of any schools that specifically teach aerial application in helicopters.