What EBrazosAg stated. Allen Academy, Brazos Christian School, and St. Joseph's Catholic school have around 20-25 seniors graduate each year. IIRC, BCS mentions in their handbook that they may not be able to offer certain classes if not enough students register and I would expect that may be the case all around. However, a private school does have some flexibility in meeting the needs of advanced students.
In College Station ISD, there are a lot of Advanced placement classes offered (I'm sure Bryan ISD has them too, I am going off of what our own experience has been).
Our children tested up to high school math prior to high school (at A&M Consolidated) and had classmates that did this for other STEM classes as well. Our children did not take dual credit classes, but focused on the AP track and took all AP classes for their regular coursework. With extracurricular activities, it is challenging, but there are certainly students who do both, or focus on academics with minimal outside activities. I know some were also taking classes at the University when they had finished the state requirements but I would have to ask my oldest how many students actually did that. Our experience was and is a positive one and we are happy with the choices we made for our family.
I know you asked for private schools, but for comparison when you are looking at schools for your student, these are the courses offered thru CSISD that are AP:
Most of the AP classes, but not all, are preceded by pre-AP classes normally beginning freshman year unless your student tests up.
English IV AP
AP Statistics
AP Calculus AB
AP Calculus BC
Biology II AP
AP Chemistry
AP Physics I
AP Physics II
AP Physics C
AP World History
AP Human Geography
US Government and Politics AP
AP Microeconomics
AP Psychology
French IV AP (language)
German IV AP
Latin IV AP
Spanish IV AP (language & culture)
Spanish V AP (literature & culture)
AP Studio Art: drawing; AP Studio Art : 2 D Design; AP Studio Art: 3D Design
AP Art History
I may have left some off, but I think I covered most if not all.
This is from the District regarding College courses that are not AP or dual credit.
Students may apply for courses through Texas A&M University's admissions office. Check with Texas A&M for exact dates. Students must rank in the top quarter of their class and have written permission to apply.
The charter school in College Station will be beginning its high school this year and there are several parents here who have students in that school. Their experience seems positive and they can probably give you a more detailed insight into what that school is like. It is definitely academically focused.
Good luck and hope the high school experience is a great one!