I bet they took it down because it looked like crap
this, OP even stated it was a joke
or
Does the school authority need to sign off on the posters or something before you can hang them? My school has a system like that, and all this hoopla might just be improper procedure.
this, my school had to have a stamp of approval, and had to be put in an approved location. Anything not following these requirements was immediately pulled down.
Free speech doesn't apply to putting things on
other people's property. That's the difference between this and the court case you cited: wearing something on your person vs attaching things to someone else's property.
Also, depending on the adhesive you use and the type of surface attached to, it's possible to damage the wall/paint. The content of your poster doesn't suddenly make that ok.