网友的猜想是有道理的，也是对的。

英语可以用 before-school 表示“上学前的”“上课前的”“课前的”。我们之所以可以在词典上查到 after-school 这个形容词，而查不到 before-school，是因为前者比较常见（所以词典收录了），后者比较少见（所以没有被词典收录）。但英语与汉语不同，大家知道，汉语是不可以随便“造字”的，但英语是完全可以根据“需要临时造词”的，尤其是一些带连字符的复合词（当然这也要遵循一定的构词法规则，而不是随意地乱造词）。先帮你查了一个 before-school 的词典例句：

The teacher’s duties that morning included supervising the before-school reading program. 那天上午这位老师的职责包括对课前阅读计划进行指导。（朗文英语写作词典）

再如大家比较熟悉的 after-tax（税后的）和before-tax（税前的）：

after-tax salary 税后薪水

after-tax income 税后收入

after-tax profit 税后利润

before-tax salary 税前薪水

before-tax income 税前收入

before-tax profit 税前利润

更多词典上查不到的复合词举例(均来自高考英语题)：

1. Everyone has taken a not-so-great class at one time or another. (2019全国卷III七选五)

2. When “Rise of the Planet of the Apes” was first shown to the public last month, a group of excited animal activists gathered on Hollywood Boulevard. But they weren’t there to throw red paint on fur-coat-wearing film stars. Instead, one activist, dressed in a full-body monkey suit, had arrived with a sign praising the filmmakers: “Thanks for not using real apes!” (2020全国卷Ⅲ阅读理解B)

3. What Theresa Loe is doing proves that a large farm isn’t a prerequisite for a modern grow-your-own lifestyle. On a mere 1 / 10 of an acre in Los Angeles, Loe and her family grow, can and preserve much of the food they consume. (2015湖北卷阅读理解B)