I have never been able to do that.
This Keep Calm and Focus On (in black & white) display printable is featured both with and without the thin line 🙂
I’m not sure if your school has cinder blocks or other 1950’s-style walls. Our walls even seem to “sweat,” which makes hanging anchor charts even trickier.
I have heard that painters tape and hot glue works wonders, but I haven’t needed to go that route yet. I’ve been using this “magic” for years with excellent results: