Are existing licences taken into consideration with further purchases?

Definitely!  The quantity discount is based on the total number of licences purchased by a single school or organisation account.  (Please note that this does not apply to Bulk Buyer Accounts.)  So, if a school with no licences purchases 1 licence, they will be charged at the single licence rate.  If in a later order they purchase 2 further licences, this will bring the total number of licences held by that account to 3, which means that the second 2 licences will be charged at the 2-5 rate.  

This applies whether the licences were purchased from AB Tutor or from an Authorised AB Tutor Reseller or from both at different times.