Add her on Snapchat using her username and wait for her to add you back. After she adds you back, she'll recognize that you're interested in her and might even snap/text you first.
Post a few private/public stories for her to see and recognize. If you know any hobbies/interests that you share with her, try to post stories about those. Don’t post too many stories, 2-4 stories is perfect so she can notice you and maybe even reply to the stories.
After you've added her and got her to see your stories, now's the time to send your first snap to start a conversation. Don’t try too much to impress her, but don’t make it too boring either. Don’t make her feel like you're sending the same snap to 20 other girls.
When she replies, keep it chill and have short conversations about her or her snaps. Send casual updates about your day without making it a big deal. The more you snap each other, the more comfortable and confident you should be.
Start talking to her regularly, in Snapchat AND in school. If you find it hard to start conversations in real life, start slow. Say hi, ask for a pen, or talk about homework.
As you start talking to her more, lightly flirt with her by complimenting her from time to time. Be genuine. Think, are you complimenting her because it’s true and genuine, or just for the sake of complimenting?
After talking for a few weeks, casually suggest hanging out. It doesn’t have to be a one-on-one date. Make it casual, like meeting up with a few friends.
Keep sending snaps and responding regularly. Talk to her in school so she understands you want to be more than just friends. Follow up after hangouts to maintain the connection.
Once you're comfortable, suggest a one-on-one date. Keep it casual and low-pressure. Pick an activity you can both enjoy.