Not only can you lock your district's grade books at the school level, you are now able to lock grade books at the grade level too.
To do this, navigate to the System Manager and open the District tab. Then, follow this path:
Grade Book Settings > Scales & Periods > choose a school year > Grade Period Sets > District Marking Period > Grade Level Locks
Once you've opened the Grade Level Locks tab, choose a school and select the Add Grade Level Lock button. To the right, select the grade level and marking period you would like to lock. Then click Save.