That's a D343 engine, common to the 769, 631, 988, 834, 630 , 983 and 657 rear engine of the '60's up to the mid '70's, they were an excellent genset and marine engine and were used un trucks as the 1693 engine.
I'm assuming that because it was owned by a school district it was well maintained and run very little.
Dennie
School districts are not know for good maintenance of equipment but you are probably right that it had little use.