I don't have the list of students yet, can I start on my timetable?

Yes. Until you have the students in the base, you can enter the approximate number of students expected per class: go to display Timetable > Classes > List, then fill in the column Entered population (Ent. Pop.). This is particularly useful for allocating appropriate classrooms.
Once students have been assigned to classes, they are automatically counted in the column Number of Students (No. Stud.), but as long as it is greater than zero, it is the entered population that is taken into account.