Remember what you're ex's schedule looks like in school.
Don't go to sports events that your ex plays in (such as their basketball game that weekend) Try to avoid places your ex always hangs out. If he/she likes bowling, only go bowling once in a while.
Block him/her on social websites and everywhere else. A few examples include Facebook, Twitter, Snapchat, Instagram, and Kik. If possible, block him/her on your phone, too.
If all else fails, tell an adult.
☛Ignore all of your ex's attempts to talk to you.
☛Don't tell your friends that you are going to avoid your ex. They might mention your plans accidentally to your ex.
☛Be careful when talking to your ex, if you talk to him/her at all.
☛Ask a teacher if you can change your seat or even tweak your schedule.
☛Don't flirt back; you don't want to get him/her more attached.