Education Secretary William J. Bennett, a frequent critic of low standards in America's schools and colleges, announced Monday he will leave President Reagan's Cabinet in mid-September. Bennett, often a magnet for controversy during his 31/2-year stint, signaled last month that he intended to step down in the fall, a few months before the Reagan administration draws to a close. Bennett told the staff he had recommended that Linus Wright, the undersecretary of education and a former superintendent of schools in Dallas, be nominated to fill the post for the final months of Reagan's term. There was no immediate word from the White House, however, on who Reagan would nominate.