"[i]f what is made available by this system (which is what is to be maintained and supported) may properly be said to constitute an education, the constitutional mandate is satisfied. Interpreting the term education, as we do, to connote a sound basic education, we have no difficulty in determining that the constitutional requirement is being met in this State, in which it is said without contradiction that the average per pupil expenditure exceeds that in all other States but two" ( id. at 48 [emphasis added]). We were careful to register our reluctance to interfere with the Legislature's funding allocations among competing imperatives by mandating an even higher priority for education funding "in the absence, possibly, of gross and glaring inadequacy" ( id. [emphasis added]).