FIRST, you need a computer expert to find out as much as pos- sible about this site and identify the service-provider. An approach to the latter would hopefully be enough to get it closed down.
Second, you sould consult the police, as it seems the perpetrators are conmitting a criminal offence.
If they are your students and you can discover who they are, it may be necessary to give them a good fright, by having the potential legal consequences of their behav- iour spelt out to them, as well as the other sanctions you may decide to impose. The full involvement of their parents in the process might be helpful.