I've seen this in quite a few places, I'm just wondering if it's a cultural myth, or whether it actually has a basis in fact.
This presentation, I think, will answer most of your questions about the inscription.
In short: Plato probably didn't inscribe anything himself. Was there something inscribed above the academy? We don't know. The inscription story began in the 4th century when it was mentioned directly by Julian the Apostate in 362 and tangentially by Sopater of Apamea. So the earliest records of such an inscription are dated more than 700 years after the academy was founded - which should speak to the lack of their gravitas.
Of course, it's a nice story :)