Normally grade 8 is indeed middle school or the final year of grade school in the US, but the Archdiocese of New Orleans is a little odd in that respect; most of their grade schools now go PK-7, so 8th graders go to the high schools, though technically under state law they are not actually *in* high school. (In the US, high school has specific requirements set by each state to qualify for a state diploma to be awarded when the requirements have been met at the end of Senior Year. In Louisiana, as in just about all US states, the work that counts toward that diploma begins in 9th grade and customarily takes 4 years to complete.) ABDMom is correct that your kids are likely to be individually evaluated for proper grade placement because the Australian school calendar doesn't at all line up with that of the US. My guess would be they would most likely be moved up a half-year. (Although, this being the US South, there is a pattern in some places with boys of so-called "redshirting"; the practice of waiting to start them in school until age 6 so that they will be larger and stronger as a sporting advantage in high school. Girls are almost never red-shirted in that way, though some parents also might redshirt children of either gender if they feel they are not yet behaviourally ready for a full day of school.)
I'm not fully up on Archdiocesan financial issues in New Orleans at the moment, but like RC schools throughout the US, they are dealing with declining enrollment. I do know that several parish grade schools were closed after Katrina because of flood destruction, and my guess is that they moved the 8th graders to the high schools in order to relieve overcrowding at the consolidated grade schools.
The distribution of how many schools you'll have to visit each day might depend on where you send them. You could, for instance, send the boys to Holy Cross and the girls to Ursuline, and they would just stay at those schools the entire time you're in the States, because both schools are now PK-12. However, in the case of Holy Cross, that still doesn't mean a single building; they have split the school into two campuses, one serving grades PK-4, the other divided into Middle/High and serving grades 5-12. Ursuline, however, is HUGE, and has everything from nursery daycare to 12th grade on one campus. (Ursuline, btw, is actually the oldest Catholic School and the first school for girls in what is now the United States; it was founded in 1727, and the only time it has ever closed was for 4 months after hurricane Katrina.)
OTOH, if you were to decide geographically and chose to live near the Lake to ease your husband's commute, the closest high schools would be single-gender; Holy Cross or Brother Martin for boys, Mt. Carmel for girls, but there are several decent co-ed RC grade schools on that side of town. (Google "Catholic schools in New Orleans map" and you'll see all the schools, including the ones on the north shore if you slide the resulting map down past the lake. The ones that are high schools show that. You'll need to zoom it up and go slow to get them all loaded.) Note that New Orleans isn't actually all that large geographically, but traffic can be slow getting across town, because there are only a few arterial roads, and the canals have bridges to cross.
If you do make the move your DD should probably start working on her dribbling, because netball really doesn't exist for kids here, but girl's basketball is everywhere, and they will be happy to have her play. The 4 RC schools best known for excellence in girls basketball in New Orleans would be Dominican HS, Ursuline, Sacred Heart (another school that teaches all ages, but all-girl), and de la Salle (co-ed, but only high school).
PS: You know, I was going to try to give you a geographical outline of the layout of the New Orleans area as we are referring to areas in this thread, but it would take too long. This website is one of the better ones I've found that explains it:
https://www.neworleans.com/plan/transportation/new-orleans-maps/
(Oh, and, it's pronounced "New ORluns" or "N'AWlins (either of those is fine), but never "New orLEENS")
