You may need some sewing experience for this one(:
Okay so you need 2 shirts they can be the same color or differtent colors doesnt matter
in the main shirt (the one that you will be wearing) cut lines horizontally on the back *DONT COMPLETELY CUT OFF THE FABRIC - STOP AT A CERTAIN POINT LIKE IN THE PICTURE*
then with the other shirt cut vertical strips but cut them out of the shirt
then weave in your vertical strips to your main shirt then only sew the tops and bottoms of the strips to the main shirt ( or you can hot glue but make sure it doesnt fall apart at school or something )
hint - try and hide your sewing stitches in the hem or something so they arent super visible
does this make sense? or confusing? I can clear it up if you want just comment below if you have any questions(: