I have 125 split up over 6 class periods

Assuming you are referring to 125 students, and you want to know how many students are in each class period, you would divide 125 by 6:

125 ÷ 6 = 20.83

Since you can't have a fraction of a student, you would round up or down depending on the situation. If you want to keep the class sizes as equal as possible, you would round down to 20 students per class period. If you want to make sure each class has at least 21 students, you would round up to 21 students per class period.