I recall that, when working at a music festival as a high school and college student, the most obnoxious and entitled type A men could be made to stop pushing (not personal questions, rather early admission or access to restricted areas) by referring to either “the insurers” or “fire safety”. Everyone would just start nodding sagely, it was amazing. “Bomb safety squad” was also a good one (there actually was this cop who would run their incredibly cute sniffer dog around the venue before every concert. We loved it. Oh happier times). No one wanted to interfere with bomb safety!
I’d think that with personal questions, a reference to strict data protection laws the college/admissions/FA office must follow, which prohibit discussing individual cases, even their own, might do the trick. Many parents will not respect a student’s boundaries when told to humorously, sarcastically or even aggressively, but they can’t argue with a faceless law.