The author of the article that I linked seems to accept the teenager's line of argument (he's no longer a teenager, of course, given how long these things take to work their way through the courts) by labeling the sign "a nonsensical phrase" and "meaningless." Quoting the young man at issue:
"What the banner said was, 'Look here, I have the right to free speech and I'm asserting it.' I wasn't trying to say anything religious, anything about drugs," Frederick said in a telephone news conference from China, where he now teaches English and studies Mandarin.
I'm willing to accept Frederick's assertion that his primary intention was political speech, but it seems naive of the reporter to characterize the message as nonsensical or meaningless. Just because a message doesn't have a simple paraphrase does not mean that it lacks meaning.
It's certainly possible to read the message as urging us to take illegal drugs in order to commune with Jesus. Although that's hardly mainstream Christianity, there is a long tradition in various religions of taking hallucinogenic substances to enter a religious trance.
But that reading seems excessively literal. I doubt even the humorless principal who suspended Frederick believes that the message was intended literally. The banner primarily derives its meaning from connotation.
The pattern "X for Jesus", or its typographical variant "X 4 Jesus," is incredibly common, and constitutes a snowclone. There are too many variants to list all of them, but they seem to fall into a few different subcategories:
Group for Jesus:
Typically a plural noun.
Jews (the original, I think)
Although some of these are straightforward groups, many have take the form of an oxymoron, including the original. That, of course, lends itself easily to satire, as in "Hookers for Jesus".
Gerunds for Jesus
(i.e., an activity, in the form of a gerund-participle)
Items for Jesus
This last type seems to be less common, although it's by no means rare, and it's the category into which "Bong Hits 4 Jesus" falls.
The sign gets its meaning by allusion to a culturally common pattern. The fact that it's an unusual collocation (illegal drug use and Christianity) is perfectly in keeping with the overall pattern of oxymoron, and here is used for satirical purpose. It pokes fun at the sometimes loopy variations of the expression by taking it to a ridiculous extreme.