Absolutely stress can play a role in test performance! I'm so sorry your son is having a stressful year in his current classroom.
One thing I'll suggest - you might want to consider private testing. It definitely sounds like the teacher situation causing problems, but I'm struck by the difference your ds' situation with her vs your older children having been through the same grade with the same teacher with no issues. Chances are it's totally the teacher, but I'd want to be sure. FWIW our 2e ds fell apart with stress in 2nd grade, and it looked to us (parents) like it was all an unreasonable teacher because his teacher was giving us feedback that seemed completely unlike what we saw at home. It's possible that stress could result in poor results on the OLSAT or other tests, but it might also be due to a 2e issue too - and it might be much easier to tease out through private testing than in school, but that's just our experience.
Best wishes,
polarbear
