We have several friends who had children of various ages at SACS. They all had great things to say about it, and they've all turned out to be decent, considerate kids. Of course, that has to do with family as well as school influence.
Our children went to Cornerstone preschool about 30 years ago. I wouldn't say we had a bad experience, but when our kids were in preschool there, the pastor (John Hagee at the time) decided to close the preschool (due to bad press about the school because of an incident that occurred at the time) with no prior notice to the teachers or the parents. There were people picking up their kids and showing up on the doorsteps of nearby Christian schools looking for literal overnight placement for their children. It wasn't a good situation, and it has grown so enormous over the years (with the original pastor's son now at the helm) that it is now a megachurch school that seems kind of impersonal.
Antonian is kind of a drive from the Stone Oak area but maybe not that far if they're in the Blanco Rd./1604 area.
Another school to consider is St. Mary's Hall near 410 and Starcrest. I've heard good things about it.
Also, St. Thomas Episcopal School is an elementary school that's not far from them on Loop 1604 near US Hwy. 281. I know children who have gone there, and their parents liked it.