At schools, people still put a lot of merit on how people look and act and whether they have lots of money and so on. At uni, it's less of a bother, because when you first go, everyone is away from home for the first time and not sure of what to expect.
If you don't like the people you're at school with, then that's ok. And as for your family asking when you're going to get married, the reply to that is "as and when I find someone I want to marry, thanks". Ignore them. My family do the exact same to me, and eventually, as you get older, they do stop asking you about it.
As others have said above, you've got to put some effort in with people. Smile, and even if you don't like going out, do so every so often, and talk to people when you do. You don't have to be the life and soul of the party, but don't lag behind waiting for people to notice you. If you disappear, chances are they won't even notice you've gone, if you've not been particularly out-going.
Going to uni will help this as the clubs & societies will have socials for you to go to and meet people, but if you've not gained some skills of appearing to be outgoing before you get there, chances are that people will still end up not talking to you and you feeling like you're just tagging along.
What about joining some sort of club for the last few months you have at home? Doesn't matter what it is, just go and do something that interests you where you'll meet people, one night a week, and those folks you're meeting will be people with a similar interest to you, and if you're doing that, it'll boost your confidence and self-esteem, which will make the people at school sit up and take notice more.