Spacing still works when the material is a real lesson, not a word list
In one of the clearest classroom tests, 39 fifth-grade children were taught unfamiliar English words using both massed and spaced schedules, with each child given a list of four words [3]. Spaced learning improved long-term retention compared with massed learning [3]. That matters because the setting is a working classroom with children and real instruction, not a controlled memory experiment with adult volunteers.
Later work has carried the same question into language teaching. Studies of spaced learning for second-language vocabulary have asked which spacing schedule best supports long-term retention [2], and classroom comparisons of spaced versus massed practice in English-as-a-foreign-language settings have used immediate and delayed tests to see whether gains hold up over time [4]. The recurring finding is directional rather than precise: spreading practice out tends to help retention persist, but these classroom studies report their own outcomes rather than a number that can be laid beside a lab result.
Why nobody can yet say how big the lab-versus-classroom difference is
A direct comparison requires a study that runs the same spacing manipulation in both a word-list laboratory task and a real classroom. None of the studies here does that. The classroom vocabulary studies examine spacing in real lessons but do not compare their results with word-list laboratory findings [2][3][4], and the adaptive spaced-repetition study in a university English course compares a spaced approach with traditional review inside the classroom only, without a word-list lab arm [5].
The lab side of the comparison is also thinner than it first appears. One paper offers a metacognitive explanation of the spacing effect, arguing that retrieval failures and the constraints they impose help account for why widely spaced practice benefits long-term retention [1]. But that account is about the mechanism of the effect, not about whether the effect behaves differently in a classroom; it does not address the lab-versus-classroom comparison at all [1]. So the two halves of the question are supported by different bodies of work that have not been stitched together.
Classroom learning adds ingredients a word list leaves out
The classroom studies hint at why a lab number might not transfer cleanly. In the fifth-grade study, children learned a small set of four unfamiliar words at a time [3], which is a far cry from the long, uniform lists typical of laboratory memory work and closer to the chunked vocabulary teaching a teacher actually does. The language-classroom studies similarly deal with vocabulary that is meant to be used, tested on both immediate and delayed occasions [4], and the university study pairs spaced repetition with an academic word list inside a credit-bearing course [5].
Those differences are plausible reasons the size of the spacing benefit could shift, but the studies here do not test that. They establish that spaced practice is associated with better long-term retention in each setting separately; they do not isolate which classroom feature, such as the smaller word sets, the stakes of a course grade, or the mixture of other schoolwork, changes the effect. Treat any claim that classrooms show a bigger or smaller spacing benefit than labs as untested by this evidence.
What a learner or teacher should take from this
The practical message is stable even though the comparison is not: spreading practice across time is worth doing, and the classroom evidence supports it for vocabulary learning specifically [3][4]. Adaptive, spaced-repetition review has also been studied as a way to bring experimental memory principles into a real university course [5], which suggests the technique can be operationalized in ordinary teaching rather than only in a lab.
What remains open is calibration. If you want to know how much longer a gap should be in a classroom than in a word-list experiment, or whether the benefit shrinks when material is meaningful rather than arbitrary, these studies do not answer it. The mechanism-oriented account of spacing, centered on retrieval failures and long-term retention [1], offers a hypothesis about why spacing helps at all, but it has not been tested against classroom conditions here. The safest summary is that the direction of the effect is well supported in both worlds, while the magnitude comparison is still an open question.
Sources used in this answer
The importance of retrieval failures to long-term retention: A metacognitive explanation of the spacing effect
Proposes a metacognitive explanation of the spacing effect centered on retrieval failures and long-term retention, without comparing lab word-list and classroom settings.
What Spaced Learning is Effective for Long-Term L2 Vocabulary Retention?
Examines which spaced learning schedule best supports long-term second-language vocabulary retention and discusses spacing effects in real-world classroom learning.
Spacing effects in real‐world classroom vocabulary learning
In real fifth-grade classrooms, teaching 39 children unfamiliar English words with spaced rather than massed schedules improved long-term retention.
Investigating the effect of spaced versus massed practice on vocabulary retention in the EFL classroom
Compares spaced versus massed practice for vocabulary retention in an EFL classroom using immediate and delayed tests, without comparing against laboratory word-list studies.
Adaptive spaced-repetition training, working memory, and long-term recall: A quasi-experimental study in university students
A quasi-experimental university study found adaptive spaced-repetition training was compared with traditional review for long-term recall in a real classroom, with no word-list laboratory arm.