Hans, Can you elaborate on your request? Would you want to do this only at the start of the school day and again at the end of the school day, or multiple times throughout the day? Would this be done to all students in one swell foop, or selectively based on some attribute like grade level, class ranking, etc?
We provide many of the building blocks to accomplish this. If you are using RADIUS authentication, then the Change of Authorization (COA) message and our User Profiles and IP Firewall policies are the items you would use to build your solution. At the start of the day students will get a User Profile with associated Firewall policy to permit (or deny) access to Facebook, then at some point later in the day using COA assign a different User Profile (with the opposite firewall policy) to those students.
Please note that the RADIUS server offered within HiveOS does not permit time of day operations, so you'll need to use a third-party server such as Microsoft NPS, Steel-Belted RADIUS, etc.