So far I know of L’ville, and the Ross school in NY. Are there any others I should be looking at?
While that might be a deciding factor in determining which BS to attend after admittance, I’m not sure of the wisdom of limiting yourself in the application process.
Many boarding schools, even if they do not use the phrase “house system,” subdivide dorms into smaller residential communities.
The majority of schools I’m considering have dorms as opposed to houses, but I feel as though i would prefer a house system. I’ve almost decided on all the schools I’m applying to (I know it’s kind of early) but I want to look at a few more that have house systems 
Andover has a mix of houses and traditional dorms for housing. The largest dorm has 45 students and the smallest house has 2. I’m not sure we really knew this during the process. When visiting we saw a traditional dorm.
Concord Academy has house style dorms too.
This is not the same thing as a “house system”.
A house system is like a fraternity or sorority. You belong to the same “house” for the entirety of your prep school years.
In Lawrenceville’s case it’s 2 out of the 4 years.
Lake Forest has a house system, I believe.
I understand that, however, I am also not sure that the OP is explaining himself/herself fully. Houses have different meanings and are not always the same as house systems. As mentioned above, Andover students live in houses (or dorms), but there is no “house system;” Eton’s houses are actually what most of use would think of as “dorms.”
Andover has a cluster system. Dorms are divided into 5 clusters. Some students stay in the same cluster but some switch.
Hun has a house system also
I think Milton has a house system.
At Milton, you stay in the same dorm for all four years.
Solebury has house system. 4 houses And you stay with your assigned house all four yrs