Hi,
If you are not able to view a particular website it is likely to be a block put on by your internet provider.
Alternatively if you manage internet filters at school level, the block is put on by your network manager.
One way to check this is to try accessing a site at home. If you can see the site at home but not at school then there is a block put on by your internet service provider / network manager. Usually this is to ensure that children cannot access inappropriate sites.
One way round this is to request unblocking of sites for usage by teachers. The sites are then unblocked when a teacher has logged in to the network.
You may find a written policy on this at your school's internet service provider.
This is how most teachers get legitimate access to student blocked sites.
If teachers and students share network computers, to ensure the blocks are in force for students, it means that a teacher must log out before a student logs in using the same computer.
If teachers and students don't share network computers there is no need for the teacher to keep logging out.
This answer would apply to any site that is blocked, not just Prezi.com