Los Angeles Schools Supt. David L. Brewer has failed to persuade a veteran educator from Philadelphia to become his deputy in charge of instruction. Brewer, a retired Navy vice-admiral with no practical experience or training in public education, is looking to hire a chief academic officer. Gregory E. Thornton, who holds a similar position in the Philadelphia school district, was Brewer's choice. But Brewer has withdrawn Thornton's name because his salary demand--$275,000 a year--was too high.