Digital flashcard apps allow students to learn and practice foreign language vocabulary independently and efficiently, leaving more classroom time for communicative activities. However, words learned this way may be forgotten. Previous lab studies have shown that vocabulary retrieval practice can be optimized for long-term memory by employing longer intersession intervals, but this lag effect has not been shown in classroom conditions. The present study investigated the optimal gap between two Quizlet sessions for retaining new vocabulary. Secondary-school students (N = 96, mean age = 13.44) learned 16 novel words in an unknown language with either a 1-day or 1-week interval. Their productive and receptive knowledge was tested after seven or 28 days. Results showed that longer spacing was beneficial for vocabulary retention, contrary to previous findings reported with school-aged learners using other types of training. The effect was small, but significantly larger on receptive tests, suggesting that the lag effect depends upon the kind of knowledge being tested.