The application process for Fairfax County’s Thomas Jefferson High School for Science and Technology stalled Saturday when technical glitches affected hundreds of students who were trying to submit proctored essays by computer.

TJ, as the magnet school is known, attracts many of the brightest students in Northern Virginia. On Saturday, 1,496 applicants who had reached the semifinal stage of the admissions process gathered at area schools to complete a writing test, but all 15 testing sites — 12 in Fairfax and three in neighboring counties — were affected by technical glitches that prevented students from saving their work.

Students were given 90 minutes to write three short essays and another 30 minutes to write a long essay. Some computers froze and others failed to store work, forcing dozens of students to rewrite portions of their essays.

“Students were wild with stress,” said Beverly Jurenko, president of the Fairfax County Association for the Gifted. “We expected that this high-stakes test would make kids tense, but this was truly upsetting.”

This year, students were for the first time required to use an online portal to submit their essays, allowing admissions officers an opportunity to supervise the writing and ensure that the essays represent the students’ work and were not unduly influenced by parents. Previously, essays were accepted in paper form.

The essays are a critical part of the application process designed to provide TJ admissions officers with a glimpse of the way applicants think.

“It’s a way for them to hear the child’s voice, which has value because you’re more than just a test score,” said Debbie Ratliff, the mother of a 14-year-old eighth-grader. She said her son, taking the essay test at Poe Middle School, lost work multiple times while writing his essays Saturday. She said a proctor allowed him one change to fix a mistake related to the computer problems but did not allow it a second time when the system ate part of his final essay.

About 500 students encountered problems. Some of the computer issues forced testing to be canceled, and 148 applicants at Rachel Carson Middle School were sent home early.

“We are now assessing the extent of the technical difficulties and are deeply sorry for the problem,” Jeremy Shughart, Fairfax’s director of admissions for TJ, wrote in a letter to parents. “Again, we apologize for this inconvenience.”

The school system will notify parents by Jan. 31 with a decision about how the admissions process will continue, school officials said. The administration said it would seek to ensure that no applicant is placed at a disadvantage because of Saturday’s problems.