What are classes (grade levels).

In aSc Timetables by "the class" we understand a group of students, with mostly the same timetable during a school year .
E.g. - all students in the 5th grade are divided into six classes - 5A, 5B, 5C, 5D, 5E, 5F. Then each class can be further divided into smaller groups for some specific subjects (like Physical education is probably divided into "boys/girls"), but basically, students in one class will have the same timetable during a cycle.

The expression "the class" doesn't mean "an event, when groups of students have an education with a teacher, in specific classroom, about particular subject". For this, we use expression "Lesson" or "Section". See this article for better understanding the difference: Adding and Editing a lesson

US high schools - Grades
If there are no clear groups of kids that have similar schedules, then you can just input entire grade as one class. Something like:

Then you can divide each grade into groups or use individual students timetables.
For understanding divisions and groups - please see these: Divisions
Students from the entire grade are divided into groups
How to input Options - each student needs to select one course from Options1 and one from Options2

Individual students (grammar and high schools)
For certain schedules it is necessary to input individual students. Each student will belong to exactly one class. So for example John from 4A or Tiffany from 9th grade.
Then you can add their "requests" for certain subjects (courses) and define specific lessons called "seminars" . Please, see this topic to understand this approach:
Student based timetables

Notes:
- It is possible to freely combine both approaches in one timetable - some lessons can be defined for classes, some for divided groups and some lessons for individual students.
- you can also create lessons that will be for joined "classes".