The top executive of the Pennsylvania School Boards Association is leaving later this year to become executive director of a similar national organization based outside Washington. The National School Boards Association announced on Monday that it has hired Tom Gentzel and that he will start work in December. Gentzel began working for the state school boards' organization as a lobbyist in 1980. The national group, with headquarters in Alexandria, Va., has nearly 100 employees and a budget of more than $20 million. It represents more than 90,000 local school board members.