Chicago State University officials have been boasting about improvements in retention rates. But an investigation by The Chicago Tribune found that part of the reason is that students with grade-point averages below 1.8 have been permitted to stay on as students, in violation of university rules. Chicago State officials say that they have now stopped the practice, which the Tribune exposed by requesting the G.P.A.'s of a cohort of students. Some of the students tracked had G.P.A.'s of 0.0.