At the largest migrant youth shelter in the country — a former Walmart in Brownsville, Tex., where the nonprofit Southwest Key Programs houses and educates 1,500 boys aged 10 to 17 — officials made it a point to show off the facility’s classrooms on a media tour in June.

Parts of the shelter resembled a typical school: brightly lit, white-walled rooms with white boards and rows of desks, and long hallways decorated with elaborate murals. One hallway displayed a giant construction paper cutout of a tree — an illustration of the three branches of government. Part of the curriculum at the shelter at the time focused on American civics and government, to coincide with the Fourth of July holiday, but there seemed to be a permanent emphasis on American patriotism and geography.

One wall in the cafeteria showed a map of the country, with the outlines of the states in bold colors. Elsewhere, the walls were emblazoned with images and quotations in Spanish and English from American presidents, from Lincoln to Trump.

The Southwest Key officials on the tour were proud of the shelter’s educational focus. But there are more than 100 facilities across the country where migrant children are detained — some run by nonprofits such as Southwest Key, others by private prison companies and government agencies — and the overall quality of the education they provide largely remains a mystery because much of what happens in the shelters is rarely seen by the public.

At Berks County Residential Center, an ICE facility in Pennsylvania, there are two classrooms, one for children aged 2 to 11 and another for children 12 to 18, according to Eleanor Acer, of the nonprofit Human Rights First. Ms. Acer, who has visited the center several times, said that the wide age span left the older children in each group bored, and that much of the instruction was done through computers and worksheets.

She added that some teachers were unable to communicate effectively in Spanish, and that classes cycle through the curriculum every two weeks, meaning students who stay longer repeat the same material.

“The impression is that they are not really taught much of anything,” Ms. Acer said.

Adrian Smith, a spokesman for ICE, said that teachers at Berks are either certified in English as a second language or working toward such a certification. Children are grouped across age ranges, he said in a written statement, because of the “varying academic abilities of students.”