You don't. You imply it.
Ignore you see in TV or movies. It's called fiction for a reason.
Here's how it works: You don't just admire her from a far, you get to know her. If you just develop a crush on her and suddenly spring into action by confessing your feelings, you end up looking creepy, and will get brushed off.
Women are very quick to categorize you as either a friend or a potential mate. First impressions are everything. By this I don't mean apperance or other such nonsense. I'm referring to the fact that you should let off to her from the beginning that you are interested in her and that you are her type. You should try to get to know her. Introduce yourself and strike up a conversation. If you can't carry a conversation, you will fail. Smile, nod and listen. Common interests are great, but not necessary. The key really is to let her do most of the talking. Don't rant on about your obsessions.
You should monitor how she acts. If she does like you, you'll be able to pick up on it and run with the ball. Parties and dates are better for this than meeting at the school cafeteria. Does she smile back at you? Does she lean towards you? Girls do a lot of subconscious things when they fancy you. You will eventually learn to pick up on these signs.
Act relatively fast. I've fucked up many times by being too timid to act. Strike when the iron is hot; go for the goodbye kiss. Never decline if she asks you in for a cup of coffee or a nightcap or whatever.
I stated that girls are quick to categorize you as a friend or potential mate. What I neglected to tell you is that they are also quite fast to demote you from a potential mate to the friend zone. When this happens, quit pining and move on. When she starts talking to you about other guys (namely about this boy she fancies), don't hang around waiting for something to happen. It's not going to, and you're just waiting for naught. Think of the friend zone as a covalent bond. Breaking it requires energy in the form alcohol and lots of sexual tension. Odds are not in your favor. Keep her as a friend (after all, she might introduce you to someone), fap a few times, but move on.