Hello
I am really sorry to hear that this has been happening to you. Telling someone you have feelings for them can be really hard. Telling them and then being treated in the way you describe is hurtful, wrong and completely unnecessary. You don't deserve this and it's not okay for people to threaten, abuse and humiliate you.
Thankfully there are a few things you can do to help the situation - with regards to what is happening at school I would strongly encourage you to talk to a teacher about this - there may be a designated teacher who deals with problems like these and you can find that out by talking to your form teacher or head of year. Every school should have an anti-bullying policy and they should take action to make sure you are safe when you are there. That includes your space and possessions being respected.
With regards to the phone calls and messages you can do one of two things, or you may decide to do both - one is to talk with your phone provider and get the numbers blocked so they can't contact you and/or you can save your messages and take them to the police. I realise that may sound scary, but threatening you can be a crime and it's important that you are safe and that the people who are calling you learn how serious that kind of behaviour is. Vandalism is also a crime and people can sometimes forget that and think it's not as bad because it happens in school. I want you to know that it is serious and you don't deserve this and you can get help.
Facebook and Twitter should have some way of reporting abuse and again, I would encourage you to try to do this.
Some people who have been bullied and terrorised in this way can find it hard to trust others and build other friendships. For them, it can take time to realise that there are good people and good friends in the world too. It's important to think carefully and seek out the right support to help you deal with what has happened. There is more information and advice on dealing with bullying on www.bullying.org.uk. It is possible to get past this, but remember you don't have to do this on your own.
I'm not sure if you have tried any of the above, if you have and feel that they may not be an option for you; please do think about talking to a ChildLine counsellor. They will listen and help talk you through your options and work towards a solution.
You are not spineless - it takes courage to tell someone how you feel and it takes courage to look for solutions in a difficult situation. You can get through this.
Take care
Sam