looking for scientific studies on using visual aids

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Anyone have any sources that cite using visual aids for kids on the spectrum and with ADHD? My sons school is reluctant to use visual aids for my son. He is in full time gen ed kinder with only 1/2 hr of special ed each day to work on IEP. His disability category is autism:PDDNOS. He is medically diagnosed with PDDNOS and ADHD. He has had no FBA but has been having difficulty with following directions and behavior lately. I suggested visual schedule earlier in the year but the teacher refused. She suggested today he may need a FBA now which I am not opposed to. He had been in PPCD the past 2 years and the school thought he would go into kinder just fine and have no problems. But now thats not the case. I need some scientifically backed research showing that PECS and visual aids are useful to show the teacher. And any other studies on methods to use for ADHD. Thanks..

Lorree