I don’t think it has much to do with public schools, as @CateCAParent mentioned only the most motivated take the SSAT. A’s in middle school are somewhat like candy, handed out almost universally. The range of kids in our kids public middle school who got A’s was pretty funny, clearly not all working at the same level. So basing how your kid will do on grades can be a bit challenging.
The SSAT is a strange test. No prep is a hard way to take it.
Have her prep and take it again.