CVAR (Corinthian Vintage Auto Racing) - Specialize in old car series. Mostly all classic and original. Maybe look into them to see what they do about modified cars.
The Driver's Edge - Generic driving school/club. Driven with them a few times. Good instructors and good people running the cars
PCA (Porsche Club of America) - Obviously more Porsche guys but good club and good track days.
Each track typically has a track owned driving school. I drove mostly at Texas World Speedway (TWS) and theirs was the Motorsports Club (MSC). Most these clubs put on events around Texas at various tracks. Lots of tracks in Texas so should be a good time for you
WELCOME!