7000 divided by 70 (using this as though all age groups were equal in size and no person over 70 lives in the community) is 100. 100×4=400 (3A enrollment). Dont know the official total for Hitchcock. Not being a smart A#$, I promise. City limit population and what's counted for population in school districts can be very different. In this case, maybe most of Hitchcock's school population lives in the designated census area for the town. Just speculation on my part.
One example. Kirbyville has a population right at 2000. If the high school has 400 students, 1/5 of the entire population of the town would be high school students. Taking it further the K-12 population would be 1300 (100 per grade). Only 700 people in the town would not be in school. Rural area population and school size are very odd.