Are you sure it's definitely dust mites only? I'm also allergic to grasses, moulds and pollens, so it's always difficult to say what causes my sniffles. I am also worse with the change of seasons. And nights are worse than days. Carpets, bedding, lounges etc can all carry dust mites, so while a pillow/mattress protector help, there are heaps of other places they'll still be. Classrooms are a haven for dust mites, esp if there are cushions in a reading corner, bean bags. Carpets in schools are always old, aren't they?! I was also worse with too much chalk dust (dustless chalk? Ha!), so basically any small particles. There could also be certain trees at school that make things worse.
What do you do if your are allergic to all these? are you taking meds?