You can remove your students from the classes by clicking on the class > check the Select All box > Remove. This will remove your students from this class - if you want to reuse the Class name.
You can click on Classes > check the Select All box and delete all your classes.
Just know that deleting classes and all students will also remove the data associated to those classes.
You will also need to make sure that you use all new student IDs as these are unique to each student and you are not able to reuse the student ID.
Please let me know if you have any other questions.