Stay calm - The most important thing ever and don't worry.
Alert authority - Doesn't have to be your mum, websites will have report buttons, you can tell your teachers, there's always teachers that different students are especially close and honest to.
Lockdown - Doesn't sound like a very bad idea but I do it myself to ensure I'm safe sometimes, close down/deactivate/delete all of your online user accounts and don't talk to anyone except those you trust most, this one is mostly for very serious cases so only if you're finding it so unbearable.
Stay calm again - If they contact you face to face, walk away or if they touch you, walk away.. Just walk in to a shop or any other busy place where there is any more than 5 people and just say to someone 'random' about the problem and what is happening briefly, if they threaten you face to face, just take it calmly and sweetly and don't say anything stupid (like you would ;) ) and walk away.. Bullies and haters hate it when your nice to them, they have a dark nature and can usually be insulted, more than anything, by kindness (sounds soppy but true enough)
I hope I helped.
Stay safe kiddo, any more problems email me at r8.productions@yahoo.co.uk
Peace,
R.