Hazel is in flower at the moment (April). Pine flowers in May. Any tree with catkins would fill the bill. Keep your eyes open for pussy willows.
Yew will be in flower soon (dioecious), and ash and oak are also wind pollinated.
The silk tassel tree has just finished (Garrya). A good tree to buy for the school grounds – very attractive. You will find information about Garrya here :- https://en.wikipedia.org/wiki/Garrya
I’ve also used plantains when available for such practicals and the yew tree in the local churchyard looked as though it was steaming a few days ago as clouds of pollen were being shed.
Later, there’s grasses.
Ask a hay fever sufferer about their least favourite wind pollinated plant and when it strikes!