Hey there. I get lonely too, but I've learned to more or less accept and be happy with my aloneness. It's actually a lot nicer than being a social bunny 24/7.
Anywho! about your problem. Do you smile at people? Do you make eye contact? Are you upbeat or do you sulk around the school? Do you exude confidence or insecurity? Considering your situation, it's probably the negative.
If you want to be noticed you need to do things to be noticed. Or you know what, fuck people. Do what you like to do, and if people notice than they do, if they don't than they don't. Pursue things you are interested in. Be your own person, stand up on your own two feet and be happy with you. Naturally, once you are doing things that you enjoy, your confidence will increase, and you will probably get noticed more. You will be doing things you like and you will meet people who have like interests, and it will make it a lot easier to talk to people.
I don't know if any of that sounds off, or weird, but you know. Just be positive. People come and go, friends come and go, they're not everything. You're 18... a lot of guys who are 18 don't have girlfriends. Hell, there are some people who are 30.. 40.. who've never had girlfriends. You're gonna be okay. Just do the things you enjoy doing, and along the way, people will come in and out of your life. Just cause they're not there now, doesn't mean they will never be. And just cause you want them now, doesn't mean you have to have them. Get comfy with being alone. It will help you out a lot.
Uhh yea. good luck! Stay positive!
