If a girl in your high school likes you, the overwhelming odds are that she won't tell you. Rather, she'll act differently towards you and give you subtle, nonverbal signals that you need to pick up on. If you don't pick up on these signals, she'll likely assume you don't like her and move on. So, if you think a girl may be interested in you, pay attention to how she acts and see if you can use this information to figure out if she likes you or not.
Take a step closer to a girl when you're talking to her. This obviously doesn't mean that you should stand so close that your lips are almost touching but rather stand just a little bit closer than you would with your friends. If she doesn't step away, she's probably at least a little bit into you.
Listen to see if she laughs at all your jokes. Not all your jokes are funny. This is just a fact. However, if a girl in your high school likes you, she will think all your jokes are funny, even the lame ones that fall flat.
Listen to see if she subtly asks if you have a girlfriend. She may do this by mentioning your supposed girlfriend in passing and waiting to see if you correct her, or she may flat-out ask. If she's interested in whether or not you have a girlfriend, she's interested in you.
Pay attention to whether she touches you or not. Some girls are just more touchy-feely than others, but if a girl constantly touches you in a friendly way while also exhibiting the signs mentioned above, she's probably interested.