You can argue Constitutional law all you want, but the fact is that it's been ruled time and again that schools can limit the wearing of such attire when it's deemed "disruptive" to the classroom environment. Sexual and drug references, funny or not, are pretty much "automatics" in schools... and always have been. In the early 70's, my parents gave my uncle a souvenir that got him sent home from school on day. They went to the Virgin Islands and brought him back one of the "I slept on a Virgin (island)!"... with "island" in a very small font. Funny, yes, but also inappropriate.
I actually got away with wearing a similar shirt in high school... I returned from a visit to Washington DC with a t-shirt from a local ice cream parlor. On the back was a line of dancing ice cream cones with the slogan "Eat me at Crumpets of Georgetown". I knew what I was doing, and I wanted the attention from my friends and classmates for wearing such a "cool' shirt. No teacher said anything, but I only wore it to school once and decided to not push my luck. Looking back at it all now, I realize it was pretty juvenile to show it off in school and I wouldn't have blamed the asst. principal if I had been asked to wear it inside-out or go home and change. What I did wasn't about "free speech"... I just wanted a lot of snickers from other kids for a cheap "joke".