MERIT PAY for teachers has been in the news, but not for the first time. When Warren Harding was president, many school boards were enthusiastic about rewarding better teachers with more money. By some estimates, between 40 and 50 percent of the country’s urban school districts adopted some such scheme in the 1920s. This enthusiasm cooled in the 1930s and 1940s, but revived during the school worries of Dwight D. Eisenhower’s presidency. It cooled again and then revived when President Reagan endorsed the notion a few years ago. The idea seems to provoke more controversy now than it did in the 1920s, but many state and local politicians have jumped on the bandwagon.