Best approach for an offline LLM assistant for 3 high school subjects

I want build an offline tutor/assistant specifically for 3 high school subjects. It has to be a tiny but useful model because it will be locally on the mobile phone, i.e. absolutely offline. For each of the 3 high school subjects, I have the syllabus/curriculum, the textbooks, practice questions and plenty of old exam papers and answers. I would want to train the model so that it is tailored to this level of academics. I would want the kids to be able to have their questions explained from the knowledge in the books and within the scope of the syllabus. If possible, kids should be able to practice exam questions if they ask for it. The model can either fetch questions on a topic from the past and practice questions, or it can generate similar questions to those ones. I would want it to do more, but these are the requirements for the MVP. I am fairly new to this, so I would like to hear opinions on the best approach. What model to use? How to train it. Should I use RAG, or a purely generative model? Is there an inbetween that could work better? What are the challenges that I am likely to face in doing this and any advice on the potential workarounds? Any other advise that you think is good is most welcome.