"Nineteenth Century School Days" will be presented today from 12:30 p.m. to 3:30 p.m. at the Hagley Museum and Library, on the grounds of the old du Pont mills in Wilmington, Del. The program will be held in the Sunday School building. Since most of the children worked in the mills, Sunday was the only day to attend school. Interpreters will talk about those days and visitors can write with quill pens and read from old textbooks. Admission is $8 for adults, $6.50 for senior citizens and students, $3 for ages 6 to 14, free for children under 6. It includes admission to the museum, the du Pont mills, mansion and gardens. The museum entrance is on Route 141. Call (302) 658-2400.