The reason a Rav would counsel someone to lie on such a form is because the school knows that their rules, as written, are extreme, and that the parent and student body are not following them. In fact, in many cases the school itself doesn’t follow the rules as written.
As such the terms in the contract are a guzma, an exaggeration, and because both parties know it is an exaggeration, it is not a lie to sign it even if you have no intention of keeping it literally (so long as you have intention of keeping its real meaning/purpose).
Mashul l’ma hadavar domeh, if it is common among shadchanim to lie about the ages of older singles, then it is mutar for a given shadchan to “lie” about a given single’s age because everyone in the field knows that you have to take the ages given with a grain of salt.