What's the deal?
You've been spending a lot of time with this guy for quite a while now: watching movies, studying together, hanging out at the mall. You talk all the time—phone calls, YM, texts, and Facebook chat—so much so that it's become part of your daily habit and it feels strange when you don't talk during the week. You've probably held hands by now, exchanged a few hugs, or maybe even a kiss. Everyone can see you're into each other, and it's not like you mind them knowing. But when your friends ask you if you guys are an item, you don't know what to say because you aren't—he hasn't asked you to be his girlfriend. So the question remains... Why aren't you officially together?
It's never any fun being in the gray area with a guy, but before you complain about him being shy and slow, think about the factors that could be preventing him from establishing a relationship with you. Did his last relationship end badly? If so, he could be apprehensive and afraid to start something new. You can't really blame him for being careful—nobody wants to get hurt. How well do you know each other? If you've only been spending time with each other for a few weeks, then maybe you haven't gotten to know each other as well as you should. Maybe he's waiting to find out everything about you, including the aspects of your personality you'd rather he didn't see.
Also, it might be helpful to think about what you want for yourself: Are you fine with the way things are between the two of you? Or does the label of "officially" being a couple really matter to you? Lastly, are you ready for the demands of being in an official relationship—not just exclusively dating each other, but sharing everything with one another and making decisions with him in mind too?
If, after all of these questions, you find that you're ready and you'd like to be with him officially, then maybe you can help him out a little...
How to get him in motion
You could ask him to attend a special school event such as a dance, the fair, or your prom with you. These events are definitely more special than your regular weekend date, and asking him to accompany you to them might steer him in the right direction. While the first two events could still be interpreted as just friendly invites, you definitely don't ask just anybody to be your date to the prom.
You could also invite him to hang out with your brothers or sisters. Introducing him to the rest of your family always means being willing to take things to the next level. However, you might want to keep your parents out of the picture in the early stages—that might scare him off.
If you're the bold, assertive type, then you can tell him you've been thinking about going out with this other guy (or guys, for that matter). The prospect of competition—and the possibility of losing you to someone else—should make him seriously consider making things official. You should be watching out for signs of jealousy and irritation at this point, but if he doesn't react at all or worse, tells you to enjoy yourself, then that's something different to consider.
If all else fails and your subtle tactics still don't get you a relationship proposal, you could just opt for the direct approach. Ask him what exactly the deal is with you two ("So... this thing we have between us... what is it?"), but don't be forceful or demanding while doing so ("Am I your girlfriend or not?"). Sometimes, there's just no beating around the bush.
When to step on the brakes
Just because things have been going great and you feel you're ready to make it official doesn't always mean that you should. A lot of it depends on how much you know about him, so if you've only had the chance to see his admirable traits, it might be too early to take it to the next level. Remember, guys always put their best foot forward early on, but will reveal their imperfections soon enough. Just 'cause he hasn't picked his nose in front of you doesn't mean he never will.
If you know he's been in a previous relationship (or three), make sure he's put his past behind him before getting together with him. Baggage can be a difficult thing to deal with, and you don't want to have to keep dealing with his issues about his ex over and over again. If you feel he still hasn't gotten over her (her name keeps popping up in your conversations, he keeps bringing you to their date places), then that's a definite warning flag for you.
It's also important to make sure that he wants an exclusive relationship just as much as you do. If you aren't officially together yet, then he could very well still be going out with other girls. If you hear or know that he's still playing the field even as you want to establish something, you'd better step on the brakes first. When he says you're "the only one for him," you better make sure he means it.