Entrance into the science/tech program are objective, based on a numerical calculation that takes into consideration: a) grades from all of 7th grade and the first quarter of 8th grade; b) verbal score from a test administered by Roosevelt in December of your child's 7th grade year; c) math score from the same test.
You can learn more here, including typical scores from prior years:
http://www1.pgcps.org/eleanorroosevelt/index.aspx?id=173914
The test is very short. They take ~225 of the top scorers.
I will add that your child should have a passion and real ability for science and math. The program is rigorous. Students in the program choose a major field of study and are required to take various advanced courses. All seniors are required to take a year-long Research Practicum class and complete an extensive paper and poster, complete with final presentation. My son was lucky enough to intern during his senior year, and he was even flown to Oregon to present his work at a professional conference.