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The only thing I can think to add to all this wonderful advice from our guild members is
Keep yourself actively engaged in a good cause!

If you're having an especially difficult time keeping something bad out of your head, -drown it out with good things! Do all you can to make sure a friend, a room mate, a study group, a tutor, a club, a party, appointing a group to help you organize parties, etc... someone is with you to help you avoid those thoughts.
Practice a musical instrument, learn to draw, to whistle, learn anything about whatever anyone is willing to teach you, anytime of the day before you find yourself alone. Sign up for every club you can find out about, -even if you're not interested in it. Do volunteer work at every opportunity. Don't be afraid to ask! Fill up your schedule!
If you can't keep all the appointments, it's okay. You'll catch them next time around, just do your best and that's all that matters.
It's not running away from your problems, it's shoving them out and filling up the space with something, -anything- else. And when all else fails, pop open the Book of Mormon and do a little extra studying there.

At some point, you'll find yourself having such a good time, feeling so filled with light and happiness, you won't believe what a struggle it was to avoid the bad thought in the first place.

I'm getting ready to leave on a 13 hour drive to CA, so this will be written fast, which is a warning:0) So I've briefly scanned what others have said, and so I may be repeating, but there are three points that came to mind.

First is that 1/3 of the host of heaven was cast out before this world was, and what do we have that they want? Bodies. And what does Satan, who loathes us for our bodies, want us to do? Defile our temples, which is our bodies. He has had about 7000 years to perfect his craft, and with these being the last days, he's gotten very good at deception and craft. He wants you to misuse your body. And abusing chastity is at the root of many sins, including ruining marriages. And Marriage is Satan's #1 attack. (Think of the Celestial Kingdom verses the other kingdoms. What does the Celestial kingdom have that others have not? Family.) Of course Satan wants to destroy the family. And he wants us to destroy any possible chance of having a righteous family. He attacks us singles as much as married couples because of our potential. He knows us and knows our potential. He wants to prevent us from entering the temple and being sealed with our choice companion. That's why chastity is so hard.

Second is that I've been thinking of your fear of being kicked out of BYU. I think that your should approach your Bishop. My senior year of High School my Parents separated, and not getting into the details, the Bishop told our broken family that you have to be willing to loose everything to gain everything. That year is one of the hardest of my life. We did loose a lot. We fought through it and relied heavily on the gospel. And though we did loose a lot, we gained so much more. We have been blessed because of our sacrifice. And I did graduate from BYU, but it took me seven years and going to three previous colleges. You probably don't like where I'm going with this, but Pres. Hinckly graduated from the U of U. And after spending many semesters at UVSC (now UVU), I missed UV so much while at the Y. I loved the Institute and spending all my time there, and I LOVED how it was right on campus. My point is that there are many great schools. Pray about it. It's an important decision. And school is starting up here quick, so I'm sure you've made your choice. This point didn't come accross very well.

Third is that we are a church of second chances. Not one of us walks perfectly. We all make mistakes. That's why we have the atonement. Use it! It's a glorious gift! (And what's great is that we can use it as many times as we wish.) But go to the Bishop. Talk to him. It's important now to learn how to control our thoughts. And we can. We're still in mortality, and it's 10 times easier to repent and change in this life then the next. The Bishop doesn't expect you to be perfect. And I know he loves you. But I think what's important is to learn, as hard as it is, and as many times as you'll fail at this, is to learn to control your thoughts. The Spirit of God cannot live in unholy temples, and with our bodies being a temple, the Holy Ghost cannot be with us if we don't strive to purify our thoughts. And having the Holy Ghost as our constant companion is so important.

Good luck. You can do it. And remember, no matter how many times you fall you can get back up. Christ didn't set a limit to the amount of times we can use the atonement.

Jade

I didn't mean for what I said to come across that way. I feel very lucky to have gone to BYU, because having gone to UV I know so many who tried to transfer and couldn't get in, and they were very good students. It's a blessing to go to BYU, and I had some incredible spiritual opportunities.

First, know that time heals. The more you work, the more you separate from it, the easier it will get. yes, it will take time, but the Lord will help you.

If you haven't acted upon these feelings, you don't have to go to the Bishop. Satan knows your weaknesses, and he will do everything he can to bring you down, but if you haven't "done the act" then you don't have to go to the bishop. But work on controlling your thoughts! Yes, it's hard, don't be ashamed, but you're young, and having gone through the repentance process, you already know what it's like. Satan will work on you. But fight him! Don't let him win!

Masturbating is wrong, and if you do that, it's something you have to go to the Bishop about. When you're ready to enter another relationship, necking and petting is wrong too. Holding hands, kissing, and hugging are okay.

But it's worth it! Find a picture of a temple you want to be married and take out your endowments in. If you have a picture close by put it where you can see it constantly. If you don't have a picture, download one and put that in a place you can see perfectly. And if you don't have a temple recommend, get a 1 yearer (where you can do baptisms), and keep that in your wallet. When ever you start to get those feelings, look at these pictures, at your recommend, or see the picture in your mind. There are pictures of the Celestial room, stare at these images of that until it's engraved in your mind. Is it worth sacrificing an eternity of happiness for momentary cravings? Are these urges worth sacrificing your worthiness to go to the temple? No, exaltation isn't worth messing with. Nothing is greater then that.

You have come so far! You're doing amazing! And if you do find yourself in a situation where you do need to talk to the Bishop don't be afraid. That's what he's there for. It might be scary, but think of the temple. Think of eternity. Think of the Celestial Kingdom.