Hitting the books in the summer used to be rare for Missouri school students. But it has become more common in the last six or seven years. Only about one-third of Missouri’s school districts offered summer school in 1993. Now, it’s 90 percent. Now, an estimated 236,000 students are taking summer classes. Some of the students attending classes during the summer months are taking make-up courses. But many others are taking classes for enrichment reasons.