Bellaire went through the same thing you describe for Dulles - my older sister's year (1970) there were over 4,000 kids in the school, a 3-year HS at that point. It wasn't until 1971 that the newly-built Sharpstown HS started taking a significant # of students from Bellaire. I think my graduating class that year was a tad over 1,000, sister's was over 1,200 a year earlier.
LOL. Seems to have worked for them; Plano produced Earl Campbell, as I remember!
But, if you think about it -- if there are 5000 kids in a school, there are still only 12 (or 24 if you count both offense and defense) spaces on the football team. And there are only 5 starters on the basketball team, and 9 starters on the baseball team.
What are the rest of the 2500 boys doing in their spare time?
There were only about 350 boys at Strake Jesuit when my oldest son started there. Every boy was required to be on one of the sports teams at the time. They hauled my kid out of the typing class to run cross country! Four years later, he placed second in the state in the Texas Catholic School Boys League his senior year. His record was still standing at least until a couple of years ago!
During my oldest son's junior year, 1977-78, Jesuit won first in the State (Texas Catholic School Boys League again) in every sport -- Football, Cross Country, Basketball, Baseball, Golf, Track, Tennis, & Soccer.
Some of those kids went on to have great sports careers, and some of them were never heard of again. But, they had the chance to compete and achieve something memorable at a young age. They would never have had a chance to do anything at Dulles. One of my son's classmates turned down a pro- football contract to go to Med School.
The other advantage is that my two sons received an education the equivalent of the Honors classes my daughters were getting at Dulles HS. The Jesuit basic education was as vigorous as the Dulles Honors classes. The Dulles Honors classes, however, were limited to 30 kids -- Total! My sons would never have been chosen out of their 5000 schoolmates.
Sending the boys to Jesuit was expensive and very time consuming for me -- I had to drive them and pick them up at least 3 days per week for about 8 years. But, it was worth every penny I spent and every hour I sat in traffic on the Southwest Freeway! I credit their Jesuit education for all their success.
BTW, I had a hard time finding a college smaller than UT, but larger than Dulles HS, for my daughter! We chose Baylor because it had 10,000 students which didn't seem to be as forbidding as UT with their 45,000 students. I gave up on size with the younger ones, however, and one son graduated from Texas Tech and the other daughter graduated from UT. My younger son graduated from University of Washington (we moved from Texas) and later received his MS and PhD from the University of Florida.
In those years Texas seemed to have lots of teeny tiny colleges and a couple of humongous colleges -- not much in between.