Almost all junior high school classes have pre-existing groups, daily life groups (生活班 seikatsu-han?). Homeroom teachers use them to arrange the lunch service rota and students will usually sit together in their group when eating school lunch. Therefore in English these groups are usually called lunch groups. Using lunch groups is convenient for English lessons as well. When you want the students to get into groups, don't waste time picking out each group - just ask them to form their pre-defined lunch groups.