SAN DIEGO—Anthony Rodriguez recalled sitting in a remedial math class at Grossmont College, bored out of his mind. The professor was teaching basic math skills that the 18-year-old had already learned in high school. Rodriguez was forced into remedial math by the community college’s placement test, which assesses a student’s ability to succeed in for-credit, higher-education classes. Rodriguez’s placement-test scores dictated at least a year of these low-level math courses. They cost the same as regular classes but don’t count toward a bachelor’s degree. Each week, Rodriguez watched as fewer and fewer classmates showed up. Eventually, he dropped out too....