A poster Alfie mentions seeing in a school is one that said "Only positive attitudes allowed beyond this point". The point Alfie makes about these sorts of signs or posters is that they reflect an ethos or culture of a school where it's not safe to speak up about what is bothering you. Alfie says:
Kids don’t require a classroom that’s relentlessly upbeat; they require a place where they’ll feel safe to express whatever they’re feeling, even if at the moment that happens to be sad or angry or scared. They need a place, in other words, where negativity is allowed. Bad feelings don’t vanish in an environment of mandatory cheer -- they just get swept under the rug where people end up tripping over them, so to speak. What you or I may describe as a negative attitude, meanwhile, may be an entirely appropriate response to an unfair rule, an intimidating climate, or a task that seems pointless or impossible. To exclude such responses from students is to refuse to think seriously about what may have given rise to their negativity.
... this status, like so much else in our schools and our society, is set up as a zero-sum game. If I become the valedictorian, then you can’t – and vice versa. In a competitive environment, our dreams are mutually exclusive.
All else being close to equal, I‘d be thrilled to send my children to a school whose walls featured variations on the timeless reminder to “Question authority.” And imagine if the principal’s office contained a framed print-out of this reminder from researcher Linda McNeil: “Measurable outcomes may be the least significant results of learning.” Visitors would be reassured that such an administrator understood a lot more about education than do most politicians.