Llama Tutor (https://llamatutor.together.ai/ and documentation: https://github.com/Nutlope/llamatutor) is a chatbot based on Llama 3.1 configured to support learning according to a grade level (6 US levels available).
The level of language is adapted and learning aids are generated on demand (quizzes, flashcards, questions / answers, help, etc.).
Sources cited.

Free, no accounts, unlimited, and open-source.