Unmarked crossroads
The accepted rule for unmarked junctions is fairly straightforward; no one has priority, however, this rule is not strictly correct.
At many junctions the priority is quite clear! The volume of traffic, width of the road, actions of other road users, etc., allow you to see at a glance whether you are on the 'through road'.
But having said that priority is often clear to see, the best training approach to take is 'never assume priority'.
You can explain that while it might be fairly obvious to you that the road you are on has priority it might not be quite so obvious to other drivers (and of course you might have got it wrong!).
With these ideas in mind you should teach pupils to approach unmarked junctions with the utmost caution but be prepared to take opportunities to proceed.
Note: Make sure that you are very clear about explaining rules for priority - priority does not equal 'right of way'. No one has 'right of way' on UK roads, 'right of way' is a legal term relating to property rights.
Drivers can be prosecuted for offences following an accident (or even without an accident) if they are shown to have acted without due care. If 'forcing' your priority leads to an accident or other problem you could find yourself in trouble!
Next: Step 5 - Typical problems and Checklist