Речь идёт об учениках, которые размещаются в классы, специально разработанные для изучающих Английский как второй язык. Меня интересует "double-block".

"Incoming Freshmen are enrolled in a double-block of math and, where appropriate, a double-block of English/Reading Intervention".

There are two possible answers for double-block:
1. As Freshmen in Brookline High School (Brookline, Massachusetts) we had English as the Second Language in writing and then went to ESL in reading. At the same time all English-speaking students attended regular English classes or what ever they had on their schedule. It’s the closed explanation to "double-block of English/Reading Intervention". But it was never called a double-block.
2. In other cases, double-block in BHS means that class takes two blocks (A and B for example, from 8:30 to 9:20 and from 9:25 to 10:15) on the same subject. In BHS it happens with chemistry, physics, and biology. And it's for everyone.