So what you are really looking for then is a Sub blog within blogs?
School <-- Root Application Main Blog
SubBlog <-- Teacher
Blog of SubBlog <-- Class Period
BE "as-is" You can have
School <--- Main Website (Teachers are Admin or have rights to create sub blogs)
SubBlog <--- Class Period (Teacher is Admin and Students are users for each class period)
In this fashion each teacher would have to create a subblog from the main website "blog".
So if there are 7 Periods then each teacher would be Admin to 7 Blogs.
For the Main website you got to make sure that the check box for
"Is For Site Aggregation"
Otherwise the Main Website "Blog" is going to display all Posts and etc from all Blogs with in BE.
So you will see 400 Users Posts :)
Hope this helps.
Have a great day!