The 24 fourth graders at Johnson Elementary School in Nahant will be split into two classes for the upcoming school year. Principal Diane Mulcahy said that there were two third-grade classes last year to accommodate students, but a previous plan was to split the second grade, which also had more than 20 students, into two classes and combine the fourth graders in one classroom. However, some families moved from town and the number of Grade 2 students decreased to 18. Jude Shanahan, who would have taught half the second graders but is certified to teach all elementary grades, will now each fourth graders. Classes begin Wednesday, Sept. 4.