Some doctors make a big deal out of allergies versus sensitivities. My allergist says they're all allergies.
Allergies can be acquired or lost anytime. I became allergic to grass a couple of years ago and lost my allergy to cats recently too. So, maybe it's time for a new allergy screening. The skin tests are OK, but the blood tests aren't very accurate. They show me allergic to peanut butter, which I eat all the time with no problem.
But I also get that congested feeling, maybe it's swollen membranes, when it's very dry outside or the heat comes on making it dry in the house. For that I do sinus wash, take Mucinex 12 hour, and especially use AYR gel around the edge of my nostrils at night.
Do you have a baby video monitor, or maybe borrow one for a couple of days? With some of them you can record or send signal to your cell phone and record. One of my DILs did this to show the ENT her son's awful breathing, which he did not have during the day. The doc took out his tonsils, which were closing off his airway. I'm not in favor of taking out anything that doesn't have to come out.
You could also try those Breathe-Right Strips to see if they would open the nasal passages a little.
Just in case it's a food or preservative sensitivity, which doesn't show up on allergy testing, you might want to keep a food and environmental journal. Write down everything, like food, of course, but also humidity, animal contact, activities, ... There might be something in the school too. Ask about pesticide spraying there.
Someone else will probably come along. Or try the archives. Best wishes!