Brainscape is a free app on the iPhone. I can also be acess on the computer. A teacher can create flashcard sets on his/her phone/computer then share it with her class. He/she can make his/her students make their own flashcards for an assignment. The app also has free sets of flashcards than are in every subject from Math to French. The app also has sets of flashcards to buy, but instead of buying the sets, a teacher could just make one. The app will send you reminders that say "Do you remember the word "merci"?" if you allow it. This allows you to study the cards throughout the day without actually accessing the app. The app test how well you know the words by how quickly you respond, and will show you words you struggle with more often than those you have mastered. The app also has a blog section that posts about your specific interests and other informative artivales about teaching and learning.
https://www.brainscape.com