You are still VERY young and are NOWHERE near your genetic potential. The fact that you have "tried everything" tends to indicate that you look for the easy wait out, which probably means that you are not training as hard, or as intelligently as you could.
Furthermore I doubt that your diet is optimal. When you are in school 5-6 hours a day it hard, if not impossible to get a good meal every 2-3 hours. The cafeteria diner severely lacks in protein, fast food restaurants are even worst! I'd try taking 2-3 protein shakes with you at school and drink one at 10:00-10:30, and one at 2:30 - 3:00.
If you are not gaining strength or size at 18 years of age it's either that you do not train correctly, eat enough nutritious food or don't get enough rest (like it or not, partying and drinking with your friends 2 times per week WILL hinder your gains SEVERELY).
Now, don't go on saying that you know how to train and stuff ... at 18 years of age you don't know anything yet ... but you think you do... which is a very dangerous combination. You do not have any answers, but since you think that you have 'em you stopped looking ... get it?
Read a lot on proper training (and I'm not talking about Muscle Fitness and such magazines). Learned advanced techniques for strength gains such as cluster sets, wave loading and block periodization.
Believe me YOU ARE NOT DOING EVERYTHING RIGHT, if you did, you will still be gaining at your age.