not exists

select buslicense  from bus a
where
not exists
 (select 1 from TkSerialSch b
  where b.s_drvTime >'2007-12-21'
  and a.buslicense=buslicense
 )


select buslicense  from bus
where
  buslicense
not in
 (select distinct b.buslicense from TkSerialSch b
  where b.s_drvTime >'2007-12-21'
  and b.buslicense  is not null
 )