In high schools that use corporal punishment in the USA can an 18 or 19 year old senior be spanked?

Yes, technically schools can, as can be found in the case of Jessica Serafin of San Antonio Texas in 2004 who later took her case all the way to the U.S. Supreme Court that ultimately refused to hear the case. Lower courts however sided with the schools-- their basic reasoning being that any student at any age who matriculates into primary or secondary schools are subject to that schools discipline policies and are not considered full adults until they graduate. Common sense would tell you though that young ADULTS (any high school age student) need mature mentoring and serious preparation for the work world they are about to find themselves in.